If you need to use a runtime-defined tr/// on more than one line, re-evaling it for each line gets very expensive. It's better to eval a subroutine into existance and use that:

#! perl -slw
use strict;

sub makeTR{
    return eval "sub{ \$_[ 0 ] =~ tr[$_[0]][$_[1]] }";
}

my $myTR = makeTR( 'abcd', '1234' );

for my $str ( qw[ abcd aabbccdd xx axbxcxdx ] ) {
    my $copy = $str;
    my $count = $myTR->( $copy );
    print "Before: $str After: $copy count: $count";
}

__END__
c:\test>junk62
Before: abcd After: 1234 count: 4
Before: aabbccdd After: 11223344 count: 8
Before: xx After: xx count: 0
Before: axbxcxdx After: 1x2x3x4x count: 4
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