my @collections = ...;

my $first_collection = shift @collections;
keys %$first_collection;
my ( undef, $max ) = each %$first_collection;
while ( my ( undef, $elt ) = each %$first_collection ) {
    if ( $elt > $max ) {
        $max = $elt;
    }
}

for my $collection ( @collections ) {
    keys %$collection;
    while ( my ( undef, $elt ) = each %$collection ) {
        if ( $elt > $max ) {
            $max = $elt;
        }
    }
}