Char Count

[Anonymous Monk]

Count chars from STDIN, takes 1 command line argument that lists all characters to be counted. RegExp does all of the work
use re 'eval';$#ARGV<0&&exit 1;$s.=$_ while<STDIN>;$s=~/(?{$z=quotemeta shift@ARGV})((??{"[$z]"})(?{$c++})|.)*(?{print$c})/m;

[jwkrahn]

You are using the /m option but you are not using either of the ^ or $ anchors.   It looks like you should be using the /s option so that . will match a newline as well as other characters.

[afoken]

$s.=$_ while <STDIN>;$s=~s/.../.../;

... lacks slurp mode and introduces an unneeded variable $s:

undef$/;$_=<STDIN>;s/.../.../;

$#ARGV<0&&exit 1;

Much typing to test if @ARGV has elements, and exit 1 can be shortened to die, if the additional error message does not disturb:

@ARGV||die;

perl -e'use re "eval";';$#ARGV<0&&exit 1;...'

... can be shortened on the perl command line:

perl -Mre=eval -e '@ARGV||die;...'

Alexander