It is true that your script will replace the Perl path within the shebang string, but it will also replace it if the same string is anywhere inside the script.

perl -pi.bak -e 's[^#!/usr/local/bin/perl][#!/usr/bin/perl]' *.pl
[download]

This one should guarantee that the replacement happens only at the beginning of a string. To ensure that the replacement is carried out only once in each script, though, one more check is needed.

perl -pi.bak -e 'BEGIN{$c=@ARGV};s[^#!/usr/local/bin/perl][#!/usr/bin/
+perl] && $c-- if $c> @ARGV' *.pl
[download]

which translates roughly into

#!/usr/bin/perl -w -i.bak
use strict
our $count=@ARGV;
while (<>) {
        if ($count > @ARGV) {
                s[^#!/usr/local/bin/perl][#!/usr/bin/perl];
                $count--;
        }
        print;
}
[download]

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