#! /usr/bin/perl
use strict;
use warnings;
use File::Find;

File::Find->find(&main::wanted, '/');

sub wanted {
  if (-T $File::Find::name) {
    open ("FILE", "< $File::Find::name")
      or die ("$!: Could not open " . $File::Find::name);
    my $first_line = <FILE>";
    unless ($first_line =~ m(^\#\!\s{0,1}/usr/bin/perl\n$) {
      return 0;
    }
    $first_line =~ s[perl][perl5]i;
    while (<FILE>) {
      $first_line .= $_;
    }
    close ("FILE")
      or die ("Could not close file "
              . $File::Find::name
              . " because $!");
    # create a temp file so if we get interrupted
    # we won't lose our data
    `cp $File::Find::name $File::Find::name~`;
    open ("FILE", "> $File::Find::name")
      or die("Can't open $File::Find::name for "
             . "writing because $!");
    print FILE $first_line;
    close ("FILE")
      or die ("Cannot close $File::Find::name "
              . "because $!");
    # get rid of the temp file
    unlink ("$File::Find::name~");
  }
}