# Use strict and warnings, etc
# Read the file and hash it
my %data;
while (<>) {
  my ($k, $v) = split /\t/; # Split on tabs
  my $values= $data{$k} ||= []; # ensure we have an array ref
  push @$values, $v;  # add the value to the end of list for this key
}

# Print out the results
for (sort keys %file) { # Sort is optional
  print join "\t", ($_, @{ $data{$_} }); # Assuming you want them tab seperated
  print "\n";
}