my ($output, @lines) = '';
my ($num, $re, $file) = @whatever;
open my $fh, '<', $file or die $!;
while (<$fh>) {
    push @lines, $_;
    shift @lines if @lines > $num;
    # $output = join('', @lines), last if /$re/;
    $output = $lines[0], last if /$re/;
}
close $fh or die $!;