open my $IN, '<', 'quoteout.dat' or die "$!"; my $searching_for_G; while (<$IN>) { $searching_for_G = 1 if 0 == index $_, 'E'; die "Error: h at line $." if $searching_for_G and 0 == index $_, 'h'; if ($searching_for_G and 0 == index $_, 'G') { print "Found G at line $.\n"; undef $searching_for_G; } }