Going a step further, we can do this in a single pass: 
#!/usr/bin/perl
use strict;
use warnings;

open my $file, '<', 'data.txt' or die "Can't open file: $!@";

my @prev = ();
while (my $line = <$file>) {
    my @spl = split(/(\.[fr])/, $line);
    if (@prev) {
        if ($prev[0] ne $spl[0] || $prev[2] ne $spl[2]){
            print @prev;
        } else {
            @prev = ();
            next;
        }
    } 
    @prev = @spl;
}
print @prev;

close $file;

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In reply to Re^2: to delete a set of specific lines in a file by eye
in thread to delete a set of specific lines in a file by sugar

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