The broken pipe happens when head reads its one line and quits. That generates SIGPIPE to the system grep process. It's harmless, you can redirect local(*STDERR) to /dev/null in a suitable scope if you don't want to see the warning

Why not do that in perl? It's easy.

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 $!;
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Untested.

Update: I realized that my code didn't do exactly what OP's does. Original line commented out and correction added.