in reply to This should be easy: counting letters in a string

The transliteration does not interpolate the variable as you expect in your sample. The following illustrates the use of eval. 
use strict;
use warnings;

my @letters = qw (A B C D E F);
my $string = "AFTYUBEWTWECRTUTYIYTDDDDRYJURTHJTREEEEEFGSDFF";
my $count;
foreach my $a(@letters) {
    eval "\$count = \$string =~ tr/$a//";
    print "$count\n";
}

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You can also use a substitution in order to interpolate but avoid the eval. 
$count = $string =~ s/$a//g;

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Both methods change the string. A more flexible way to count the values would be to split the string and use a hash to store the values. 
my %count;
$count{$_}++ foreach split //, $string;
print "$count{$_}\n" foreach (@letters);

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Re^2: This should be easy: counting letters in a string
by Roy Johnson (Monsignor) on Jan 31, 2006 at 14:46 UTC
    Both methods change the string.
    No, tr/a// will not delete characters unless you specify the d modifier. Characters that are not replaced are left alone.

    And using a little trick to induce list context, you can use pattern matching to count the occurrences: 

    my @letters = qw(A B C D E F);

my $string = 'AFTYUBEWTWECRTUTYIYTDDDDRYJURTHJTREEEEEFGSDFF';

my @counts;

for (0..$#letters) {
  $counts[$_] = () = $string =~ /$letters[$_]/g;
}

print "@counts\n";

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