ApplyRegExRename('witch(\d\d\d)\.jpg','witch_%02d.jpg');

# param 1 = regex expression for find
# param 2 = sprintf expression for replacement
sub ApplyRegExRename # supports up to 5 caught matches within the pattern
{
    my $regex = shift;
    my $repl = shift;
...
        my $file2 = $file;
        $file2 =~ s/$regex/sprintf($repl,$1,$2,$3,$4,$5)/e;
        print "renaming: $file to $file2\n";
...
}