use strict;
use IO::File;

my %data; # hash to store data from first file

my $f = new IO::File "first.txt", "r" or die "Can't open file 1";
while (<$f>) {
    chomp;
    my ($login) = /^(\w+):/;
    $data{$login} = $_;
}
undef $f;

my $f = new IO::File "second.txt", "r" or die "Can't open file 2";
my $o = new IO::File "third.txt", "w" or die "Can't create file 3";
while (<$f>) {
    chomp;
    my ($login,$info) = /^(\w+)(:.*)/;
    if ($data{$login}) {
        print $o $data{$login} . $info, "\n";
    }
}
undef $f;
undef $o;

##</code><code>##

---- first.txt ----
0001:rec1:rec2:rec3
0002:recA:recB:recC
0003:recX:recY:recZ

---- second.txt ----
0001:rec4:rec5:rec6
0002:recD:recE:recF

##</code><code>##

---- third.txt ----
0001:rec1:rec2:rec3:rec4:rec5:rec6
0002:recA:recB:recC:recD:recE:recF