Quantifier follows nothing in regex; marked by <-- HERE in m/* <-- HERE **SVS***
/ at temp.pl line 13.

open(INO,"old.txt");
my @ino = <INO>;
close(INO);
open(INN,"new.txt");
my @inn = <INN>;
close(INN);

my @out;
for (my $line=0; $line < @inn; $line++) {
    my $readline = $inn[$line];
    chomp ($readline);
    foreach $old(@ino){
        if($old =~ /$readline/){ push @out,$old; };
    }
}
open(OUT,">filterd.txt");
foreach $_(@out){print OUT "$_";};
close(OUT);

$old =~ /$readline/

$old =~ /\Q$readline\E/

/\Q$readline\E/

index($old, $readline) >= 0