gistfile1.txt

my @matches;
my @filenames = (...);

for my $filename (@filenames) {
    open(my $fh, '<', $filename) or die "Failed to open $filename - $!";  
    while (my $line = <$fh>) {
        if ($line =~ /foo/) {
            push @matches, $line;
        }
    }
}