#!/usr/bin/perl
use strict;
use warnings;
my $prev1=0;my $prev2=0;my @dels;
open my $file, '<', 'data.txt' or die "Can't open file: $!@";

my %dels;
while (<$file>) {
    # the \. prevents . from matching any character
    my @spl = split(/\.[fr]/, $_);
    if($prev1 eq $spl[0] && $prev2 eq $spl[1]){
        $dels{$spl[0]} = 1;
    }
    $prev1=$spl[0];$prev2=$spl[1];
}

# reset the file cursor, read from the beginning again
seek $file, 0, 0;
while (<$file>) {
    my @spl = split(/\.[fr]/, $_);
    print unless $dels{$spl[0]};
}
close $file;