Pattern matching across two files, Need something better than grep -f!

[anasuya]

I have a pattern.txt file which looks like this:

2gqt+FAD+A+601   2i0z+FAD+A+501
1n1e+NDE+A+400   2qzl+IXS+A+449
1llf+F23+A+800   1y0g+8PP+A+320
1ewf+PC1+A+577   2a94+AP0+A+336
2ydx+TXP+E+1339   3g8i+RO7+A+1
1gvh+HEM+A+1398   1v9y+HEM+A+1140
2i0z+FAD+A+501   3m2r+F43+A+1
1h6d+NDP+A+500   3rt4+LP5+C+501
1w07+FAD+A+1660   2pgn+FAD+A+612
2qd1+PP9+A+701   3gsi+FAD+A+902

There is another file called data (approx 8gb in size) which has lines like this.

2gqt+FAD+A+601   2i0z+FAD+A+501    0.874585  0.785412
1n1e+NDE+A+400   2qzl+IXS+A+449    0.145278  0.589452
1llf+F23+A+800   1y0g+8PP+A+320    0.784512  0.341786
1ewf+PC1+A+577   2a94+AP0+A+336    0.362542  0.784785
2ydx+TXP+E+1339   3g8i+RO7+A+1     0.251452  0.365298
1gvh+HEM+A+1398   1v9y+HEM+A+1140  0.784521  0.625893
2i0z+FAD+A+501   3m2r+F43+A+1      0.369856  0.354842
1h6d+NDP+A+500   3rt4+LP5+C+501    0.925478  0.365895
1w07+FAD+A+1660   2pgn+FAD+A+612   0.584785  0.325863
2qd1+PP9+A+701   3gsi+FAD+A+902    0.874526  0.125453

However the data file is not as simple as it looks like given above. The large size of the file is due to the fact that there are approx 18000 lines in it which begin the string in the first column of every line. i.e. 18000 lines beginning with 2gqt+FAD+A+601, followed by 18000 lines beginning with 1n1e+NDE+A+400. But there will be only one such line which matches the given pattern as in pattern.txt I am trying to match the lines in pattern.txt with data and want to print out:

2gqt+FAD+A+601   2i0z+FAD+A+501 0.785412
1n1e+NDE+A+400   2qzl+IXS+A+449 0.589452
1llf+F23+A+800   1y0g+8PP+A+320 0.341786
1ewf+PC1+A+577   2a94+AP0+A+336 0.784785  
2ydx+TXP+E+1339   3g8i+RO7+A+1  0.365298
1gvh+HEM+A+1398   1v9y+HEM+A+114 0 0.625893
2i0z+FAD+A+501   3m2r+F43+A+1 0.354842
1h6d+NDP+A+500   3rt4+LP5+C+501 0.365895
1w07+FAD+A+1660   2pgn+FAD+A+612 0.325863
2qd1+PP9+A+701   3gsi+FAD+A+902 0.125453

As of now I am using something in perl, like this:

use warnings;
open AS, "data";
open AQ, "pattern.txt";
@arr=<AS>;
@arr1=<AQ>;
foreach $line(@arr)
{
    @split=split(' ',$line);
    foreach $line1(@arr1)
    {
     @split1=split(' ',$line1);
     if($split[0] eq $split1[0] && $split[1] eq $split1[1])
     { print $split1[0],"\t",$split1[1],"\t",$split1[3],"\n";}
   }

}
close AQ;
close AS;

I have tried using grep -f, but it is taking a very long time to do this job. how do i modify this existing code using something like:

while ($line = <AQ>) #file handler for pattern
{
   while ($line_data = <AS>)
    { #do the matching here.?
    }
}

I want to minimise the runtime of this code to as small as possible. please help.