dynamically named subarrays based upon the 1st column's count

Yes, you want a hash of arrays.

my %snmp_who;

while(<>) {
    my ( $id, $type, $value ) = split " ", $_, 3; # !FIXME!
    push @{ $snmp_who{$id} }, $value;
}

print "@{ $snmp_who{$_} }\n" for keys %snmp_who;
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You should at least read perldoc perlreftut. perldoc perldsc and perldoc perllol also provide helpful introductions to the same topics.

Note that I don't know the exact specification of the SNMP reply format, so my split solution is likely broken.

Come to think of it there's Net::SNMP on CPAN, so unless you have a really good reason not to (and there are very very few of those), you should use that instead.