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

# We don't really need an array for one line, but it seems
# conceptually nicer (and generalises more easily)
my $max_buffer_size = 1;
my @buffer;

my $line;
while ($line = <ARGV>) {
    push @buffer, $line;

    # Replace the pattern match with your criterion if this
    # isn't right.
    @buffer = () if $line =~ /^.id/;

    if (scalar @buffer > $max_buffer_size) {
        print shift @buffer;
    }
}
print @buffer;