use Data::Dumper;
my %hash1 = (a => [1, 2], b => ['k', 'm']);
my %hash2 = (a => [3, 4], b => ['o', 'p']);
my %result;

for (keys %hash1) { 
  $result{$_} = [@{$hash1{$_}}, @{$hash2{$_}}];
}
print Dumper \%result;

#oder mit Aenderung:
for (keys %hash1) { 
  push @{$hash1{$_}}, @{$hash2{$_}};
}

print Dumper \%hash1;