Well, the words "huge file" make me wonder if this is the right way to go, but if you want one entry per key, that's spelled "hash" in Perl. Here's a sample (untested) that should take you in the right direction:

my %hash;          # crappy name but easy to remember
while (<FILE>)     # assuming FILE is open to the right place
{
    my ($key, $value) = split /\t/;   # only one tab per line
    push @{$hash{$key}}, $value;      # magic of autovivification
}
# done, print everything
foreach my $key (keys %hash)
{
    print $key, " => ", join(' ', values @{$hash{$key}}, "\n";
}
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For more information, you may want to start with the Perl data structures cookbook.

Update
BrowserUK is correct below, hashes don't preserve input order. For that, the easiest modification is to use Tie::IxHash if you can take the time and memory hit.