This peace of code does exactly what you want.

#!/usr/bin/perl
my %t;
while ( defined($_ = <DATA>) ){
  chomp;
  my ($k, $v) = split /\s+/;
  push @{$t{$k}}, $v;
}
for ( sort keys %t ) {
  my %q;
  @q{@{$t{$_}}} = ();
  print "$_ ", join( ' ', sort keys %q), "\n";
}
__DATA__
1    101
2    102
3    103
1    104
2    102
3    104
[download]