Minor tweak: since the OP wants to "delete" lines that do not match a certain pattern, I'd change the unless into an if. That way, you "save" the lines that do match the pattern ;-)

open IN, "<$bigfile" or die "Can't open $bigfile: $!\n";
open OUT, ">$newfile" or die "Can't open $newfile: $!\n";
while(<IN>) {
   print OUT $_ if /the pattern in question/;
}
close OUT;
close IN;
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