Scalar::Util's looks_like_number tells the difference between what's a number and what's not (according to Perl's definition).

use Scalar::Util qw( looks_like_number );

die("Invalid amount of Gil \"$gil\"\n")
    if !looks_like_number($gil);

die("Invalid character name \"$char\". "
   ."It must only contain letters [a-z])\n")
    if $char !~ /^[a-zA-Z]+\z/;
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