use strict;
use warnings;

my $file1=<<EOF;
23 SNP_A-4293670 0 2713391
24 SNP_A-4293670 0 2713391
25 SNP_A-1780270 0 1111111
26 SNP_A-1780271 0 2222222
EOF

my $file2=<<EOF;
SNP_A-1780270 ss75925050 rs987435
SNP_A-1780271 ss75925050 rs000001
SNP_A-4293670 ss75925050 rs999999
SNP_A-4293670 ss75925050 xxxxxxxx
EOF

my %lookup;
foreach my $line ( split( /\n/, $file2) ){
    my @line=split(/\s+/,$line);
    $lookup{$line[0]}=$line[2];
}
print "k=$_,v=$lookup{$_}\n" for keys %lookup;

foreach my $line ( split( /\n/, $file1) ){
    my @line=split(/\s+/,$line);
    printf "%s\t%s\t%s\t%s\n", $line[0], $lookup{$line[1]}, $line[2], $line[3];
}


##</code><code>##

23      xxxxxxxx        0       2713391
24      xxxxxxxx        0       2713391
25      rs987435        0       1111111
26      rs000001        0       2222222