s/$2/something/   

##</code><code>##


[root@myserver tmp]# cat perl.test
#!/usr/bin/perl -w

use strict;

my $string = "123 123 345";
if ("$string" =~ /(\d+)\s+(\d+)\s+(\d+)/) {
    $string =~ s/$2/something/;
    print "$string\n";
}

[root@myserver tmp]# ./!$
./perl.test
something 123 345