In case you want to start using an array for the entry as soon as there is more than one element (and only then), you could do

    ...
    my $key = <STDIN>;
    chomp($key);
    my $val = <STDIN>;
    chomp($val);
    if (exists $hash{$key}) {
        $hash{$key} = [ $hash{$key} ] unless ref($hash{$key});
        push @{$hash{$key}}, $val;
    } else {
        $hash{$key} = $val;
    }
[download]

Update: this would produce a data structure like this

$VAR1 = {
          'bar' => 'val3',
          'baz' => 'val4',
          'foo' => [
                     'val1',
                     'val2'
                   ]
        };
[download]

Whether that's convenient to use further on, is another question... Maybe you'd want to always use an array for each entry, in which case you could simply write (using autovivification):

    ...
    my $key = <STDIN>;
    chomp($key);
    my $val = <STDIN>;
    chomp($val);
    push @{$hash{$key}}, $val;
[download]

producing:

$VAR1 = {
          'bar' => [
                     'val3'
                   ],
          'baz' => [
                     'val4'
                   ],
          'foo' => [
                     'val1',
                     'val2'
                   ]
        };
[download]