instead of using a hash where keys and values are strings, use a hash where the keys are strings and the values are references to arrays (so you can put more than one value for a key):

my %h;
while( <> ) {
  my ($k, $v) = process $_;
  push @{$h{$k}}, $v
}
for my $k ( keys %h ) { #traverse all keys
  for my $v ( @{$h{$K}} ) { #traverse all values for that key
    do_your_stuff $k, $v;
  }
}
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and read perllol, perlreftut, perlref, perldsc... I would prefer you read them in that order, but do as you please!!!