sub colNOEXISTS () {-1}

# and, then, in the for statement...
$Columns{$col_name}{exists} == colNOEXISTS 
    and die "your error here: $!";

##</code><code>##

$i=1; $j=2; $k=3;
$l = $i++ * $j++ for 1..$k;

$i=1; $j=2; $k=3;
$m = $j++ * $k++ for 1..$i;

$i=1; $j=2; $k=3;
$k=3;