Enhancement: New SHEBANG=relocatable MM option

- Adds the option to force a relocatable shebang by setting the
  "SHEBANG=relocatable" MM option.
- Document SHEBANG=relocatable

lib/ExtUtils/MM_Any.pm

@@ -2373,7 +2373,7 @@ CODE
     $self->{UNINSTALL}   ||= $self->oneliner('uninstall', ["-MExtUtils::Command::MM"]);
     $self->{WARN_IF_OLD_PACKLIST} ||=
       $self->oneliner('warn_if_old_packlist', ["-MExtUtils::Command::MM"]);
-    $self->{FIXIN}       ||= $self->oneliner('MY->fixin(shift)', ["-MExtUtils::MY"]);
+    $self->{FIXIN}       ||= $self->oneliner('$ENV{PERL_MM_SHEBANG} ||= "$(SHEBANG)"; MY->fixin(shift)', ["-MExtUtils::MY"]);
     $self->{EQUALIZE_TIMESTAMP} ||= $self->oneliner('eqtime', ["-MExtUtils::Command"]);
 
     $self->{UNINST}     ||= 0;

lib/ExtUtils/MM_Unix.pm

@@ -429,6 +429,7 @@ sub constants {
               PERL_CORE
               PERM_DIR PERM_RW PERM_RWX
 
+              SHEBANG
 	      ) )
     {
 	next unless defined $self->{$macro};
@@ -1187,6 +1188,10 @@ WARNING
 
 Inserts the sharpbang or equivalent magic number to a set of @files.
 
+If the WriteMakefile option C<SHEBANG> is set to "relocatable", then any found
+shebang/sharpbang lines are set to C<#!/usr/bin/env perl> instead of the
+default perl.
+
 =cut
 
 sub fixin {    # stolen from the pink Camel book, more or less
@@ -1261,8 +1266,17 @@ sub _fixin_replace_shebang {
 
     # Now look (in reverse) for interpreter in absolute PATH (unless perl).
     my $interpreter;
-    if ( defined $ENV{PERL_MM_SHEBANG} && $ENV{PERL_MM_SHEBANG} eq "relocatable" ) {
-        $interpreter = "/usr/bin/env perl";
+    if ( defined $ENV{PERL_MM_SHEBANG} ) {
+        if ( $ENV{PERL_MM_SHEBANG} eq "relocatable" ) {
+            $interpreter = "/usr/bin/env perl";
+            print "SHEBANG option set to 'relocatable': Changing shebang from '$cmd $arg' to '$interpreter'"
+                if $Verbose;
+            $arg = ""; # args don't work with /usr/bin/env perl
+        }
+        else {
+            print "SHEBANG option set to something other than 'relocatable': Ignoring it"
+                if $Verbose;
+        }
     }
     elsif ( $cmd =~ m{^perl(?:\z|[^a-z])} ) {
         if ( $Config{startperl} =~ m,^\#!.*/perl, ) {

lib/ExtUtils/MakeMaker.pm

@@ -325,6 +325,8 @@ sub full_setup {
 
     MACPERL_SRC MACPERL_LIB MACLIBS_68K MACLIBS_PPC MACLIBS_SC MACLIBS_MRC
     MACLIBS_ALL_68K MACLIBS_ALL_PPC MACLIBS_SHARED
+
+    SHEBANG
         /;
     push @attrib_help, @fs_macros;
     @macro_fsentity{@fs_macros, @dep_macros} = (1) x (@fs_macros+@dep_macros);
@@ -2785,6 +2787,16 @@ A minimal required perl version, if present, will look like this:
 
     perl(perl)>=5.008001
 
+=item SHEBANG
+
+When set to C<relocatable>, it makes C<ExtUtils::MM_Unix-E<gt>fixup()> change
+any shebang lines found (e.g. C<#!/usr/local/bin/perl>) into a relocatable
+version C<#!/usr/bin/env perl>. This is useful when writing tools that are
+intended to work seamlessly in different environments, eg. both in containers
+and with C<plenv(1)>.
+
+If C<SHEBANG> is set to anything else than C<relocatable>, it is ignored.
+
 =item SITEPREFIX
 
 Like PERLPREFIX, but only for the site install locations.