use strict;                                # duh.
use File::Find;
find ( sub {
   open IN, "< $_" || return;              # silently discard failed open
   chomp(my $firstline = <IN>);            # read in first line and
   $firstline =~ /^#!.*perl[\s\Z]/ && do { # check for a shebang
      $firstline = "#!$^X$'";              # replace with your own shebang
      chomp(my @nextlines = <IN>);
      open OUT, "> $_" && do {             # print the file back out
         map {print OUT $_,$/;} $firstline, @nextlines;
         close OUT;
      }
      # the next bit is a little more concise, but won't replace newlines
      #$firstline = "#!$^X$'$/";
      #my @nextlines = <IN>;
      #open OUT, "> $_" && do {             
      #   map {print OUT} $firstline, @nextlines;
      #   close OUT;
      #}
   close IN;
   }, qw|
         /source/code/directory
         /another/source/dir
        |;
)