in reply to Entering the land of Perl

Hello manbroski, welcome to the Monastery!
Prototypes are seldom used in Perl. It is usually enough to pass a reference as the argument directly: 
sort_and_print_hash_keys(\%letters);

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Moreover, counting characters in a string is idiomatically done via the tr operator. We know in advance that the possible characters are A, C, T and G only, so you can write: 
#!/usr/bin/perl
use strict;
use warnings;
use feature qw(say);

my $s = shift;
say join " ", map eval "$s =~ tr/$_//", qw/A C G T/;

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Which was exactly my solution to the DNA problem on Rosalind :-)
لսႽ† ᥲᥒ⚪⟊Ⴙᘓᖇ Ꮅᘓᖇ⎱ Ⴙᥲ𝇋ƙᘓᖇ

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Re^2: Entering the land of Perl
by manbroski (Initiate) on Apr 04, 2013 at 21:51 UTC

    That, sir, is a beauty.

    Anything functional (in the map-reduce sense) makes me quite happy. You have my thanks.

    EDIT: what version is this? 5.16 'use strict' returns all zeros for each count. Does that provide the function return code?

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      The shift requires the string as an argument of the script. If you want to read it from a file, you have to use
      $s = <>;

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      instead and provide the input file as the argument.
      لսႽ† ᥲᥒ⚪⟊Ⴙᘓᖇ Ꮅᘓᖇ⎱ Ⴙᥲ𝇋ƙᘓᖇ
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[d/l]

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