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my $key = (split/\t/, $line)[0];

push @{ $hash{$key} }, $line;

while(<DATA>)
{
...
NM_000044    211110_s_at
NM_000044    211621_at

$VAR1 = {
          'NM_000032' => [
...
                           'NM_000036    206121_at'
                         ]
        };