use strict;
use warnings;
my %result;                             # Hash of Arrays

my $headline = <DATA>; 					    # ignore
while (my $line =<DATA>) {
  my ($gene,$sample) = split /\s+/, $line;
  push @{$result{$gene}},$sample;
}

for my $gene (sort keys %result) {
  print join "\t", $gene, @{$result{$gene}};
  print "\n";
}

__DATA__
Gene_name sample
gene_A	sample_1
gene_B	sample_1
gene_C	sample_1
gene_B	sample_2
gene_C	sample_2
gene_A	sample_3

##</code><code>##

gene_A	sample_1	sample_3
gene_B	sample_1	sample_2
gene_C	sample_1	sample_2