you want to use grep.
try perldoc -f grep

...
#don't do `cat ...`
open( FH, "<flatfile.txt") or die "Can't open flatfile.txt:$!\n";

my @matches = grep /File Error/, <FH>;
if ( $#matches > -1 ) {
        print "Got " . ($#matches + 1) . " matches\n";
}
[download]

Hope that helps.