I have to find all ready that match with no mismatch

If you're only looking for exact matches, something like this does the job in ~ 7 seconds (including building the index):

#! perl -slw
use strict;
use Time::HiRes qw[ time ];

our $MIN        //= 5;
our $SHORT      //= 20;
our $NSHORTS    //= 10e3;
our $BIG        //= 5e6;
our $S          //= 0;

$S and srand( $S );
sub rndDna{ my $dna =''; $dna .= ( qw[A C G T] )[ rand 4 ] for 1 .. $_
+[0]; return $dna; }

my $bigDna = rndDna( $BIG );
my @shorts = map rndDna( $SHORT ), 1 .. $NSHORTS;

my $start = time;
my %idx; push @{ $idx{ substr $bigDna, $_, $MIN } }, $_ for 0 .. lengt
+h( $bigDna ) - $MIN;
printf STDERR "Indexing took %.9f secs\n", time() -$start ;

$start = time;
my $found = 0;
for my $short ( @shorts ) {
    for my $pos ( @{ $idx{ my $pre = substr $short, 0, $MIN } } ) {
        if( substr( $bigDna, $pos, $SHORT ) eq $short ) {
            print "$short found at $pos";
            ++$found;
        }
    }
}

print STDERR "Found: ", $found;
printf STDERR "Lookup of $NSHORTS $SHORT-nt short reads against a $BIG
+-nt sequence took %.9f secs\n", time() -$start ;

__END__
C:\test>1120869-2 -BIG=5e6 -MIN=5 -SHORT=14 -NSHORTS=10000 -S=1 | wc -
+l
Indexing took 3.365217924 secs
Found: 182
Lookup of 10000 14-nt short reads against a 5e6-nt sequence took 25.56
+8705082 secs
    182

C:\test>1120869-2 -BIG=5e6 -MIN=6 -SHORT=14 -NSHORTS=10000 -S=1 | wc -
+l
Indexing took 3.351562023 secs
Found: 182
Lookup of 10000 14-nt short reads against a 5e6-nt sequence took 6.666
+512966 secs
    182

C:\test>1120869-2 -BIG=5e6 -MIN=7 -SHORT=14 -NSHORTS=10000 -S=1 | wc -
+l
Indexing took 4.609364986 secs
Found: 182
Lookup of 10000 14-nt short reads against a 5e6-nt sequence took 1.699
+807882 secs
    182

C:\test>1120869-2 -BIG=5e6 -MIN=7 -SHORT=14 -NSHORTS=10000 -S=1 | wc -
+l
Indexing took 4.519516945 secs
Found: 182
Lookup of 10000 14-nt short reads against a 5e6-nt sequence took 1.682
+117939 secs
    182

C:\test>1120869-2 -BIG=5e6 -MIN=8 -SHORT=14 -NSHORTS=10000 -S=1 | wc -
+l
Indexing took 6.175763845 secs
Found: 182
Lookup of 10000 14-nt short reads against a 5e6-nt sequence took 0.424
+978018 secs
    182

C:\test>1120869-2 -BIG=5e6 -MIN=10 -SHORT=14 -NSHORTS=10000 -S=1 | wc 
+-l
Indexing took 9.214743853 secs
Found: 182
Lookup of 10000 14-nt short reads against a 5e6-nt sequence took 0.058
+255196 secs
    182
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Note: I'm using 14nt short reads because any longer and none are found in my randomly generated test sequence.

You can reduce the lookup time by increasing number of characters you index; with the trade off that the indexing take longer and uses more memory:

1 x 5e6nt sequence; 10e3 x 14nt short reads; 
Index prefix:   indexing(secs):   lookup(secs): found:   memory(MB):
 5              3.37              25.57         182      200
 6              3.38               6.66         182      210
 7              4.06               1.69         182      230
 8              6.17               0.42         182      244
10              9.21               0.05         182      450
[download]