A little starting point:

#!/usr/bin/perl
use strict;
use warnings;

print "Starting external program. Waiting for input.\n";
open my $fh, "-|", "while true; do date; sleep 1; done";
my @lines;
while(defined(my $line = <$fh>)) {
    chomp $line;
    push @lines, $line;
    if(@lines >= 20) {
        my $howmuch = @lines;
        print "Read '$howmuch' lines from external program.\n";
        print "Stats are...calc calc calc.\n";
        print "The following lines where read:\n";
        print map { "$_\n"; } splice @lines;
        print "Going to sleep.\n";
        sleep 20;
    }
}
[download]

Looking at the docs of the used commands should push you forward. Search here helps also (reading from external program, capturing from external program).

Best regards
McA