#!/usr/bin/perl -w

use strict;

my @files = `find . -name "*.pl"`;
foreach my $file (@files) {

    local*FH;
    open(FH, ">$file");
    my @contents = <FH>;
    $contents[0]="#!/usr/local/bin/perl -w";
    print FH $_ foreach (@contents);
    close FH;

}
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Yup, I'm aware of the bad practice in this script, however, its just a quick hack.

NB: I've taken the liberty to add warnings (-w) to your shebang, and it may break your scripts, although, it shouldnt.. :-)

Update: Fixed typo as per defyance's comment. Please note the "untested" disclaimer.