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

my $file = $ARGV[0] or die "Usage: $0 <input_file>";
open(my $fh, '<', $file) or die "Unable to open '$file' for reading: $!";

my ($curr_key, @rec) = ('', ());
while (<$fh>) {
    chomp;
    my $entry = parse_line($_);
    if ($entry->{key} ne $curr_key) {
        process_rec($curr_key, \@rec);
        ($curr_key, @rec) = ($entry->{key}, $entry);
    }
    else {
        push @rec, $entry;
    }
}
process_rec($curr_key, \@rec);

sub parse_line {
    my ($line) = @_;
    my %entry;
    # ...
    return \%entry;
}

sub process_rec {
    my ($key, $rec) = @_;
    return if ! @$rec;
    # ...
}