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;