use strict;
use warnings;
use autodie;
my $simulation = \do{
       'field 01' . ','x79 . 'field 80,' . "\N{LF}"
      .'field 01' . ','x79 . 'field 80,' . "\N{LF}"
};
*OUT = *STDOUT;


my $OFFSET = -2;        # second last chracter (the ',') in $line
my $LENGTH =  2;        # replace 2 charactrers (',' and  \n)
my $REPLACEMENT = '';   # replace specified characters with nothing

open IN, '<', $simulation;
while(<IN>) {
    my $line = $_;
    my $number = () = $line =~ /\,/gi;
    substr($line, $OFFSET, $LENGTH) = ''  if ($number > 79);
    print OUT "$line";
    }    
close OUT;
close IN;