my @common = grep { exists $bar{$_} } keys %foo;

Update: ++jlf for sharp eyes and kind acts. Typo repaired.

Update²: ++maverick for the heads-up and the good solution. Here is another way to do it as a function with a list of hashrefs:

sub commonkeys {
    my %common = %{-shift};
    for my $hr (@_) {
        delete @common{ grep { !exists $hr->{$_} } keys %common };
    }
    [keys %common]
}
[download]

This doesn't do much copying. Returns an array ref to keys common to all.

After Compline,
Zaxo