Here's the whole function that contains that line, with line 150 commented like "# LINE 150":

sub release
{
    my ($self, $force) = @_;
    my (@tofree);

    return($self) unless(ref $self);
# delete stuff that we know we can, here

    if ($force)
    {
        foreach my $key (keys %{$self})
        {
            push(@tofree,$self->{$key});
            $self->{$key}=undef;
            delete($self->{$key});
        }
    }
    else
    {  @tofree = map { delete $self->{$_} } keys %{$self}; }

    while (my $item = shift @tofree)
    {
        my $ref = ref($item);
        if (UNIVERSAL::can($item, 'release')) # LINE 150
        { $item->release($force); }
        elsif ($ref eq 'ARRAY')
        { push( @tofree, @{$item} ); }
        elsif (UNIVERSAL::isa($ref, 'HASH'))
        { release($item, $force); }
    }

# check that everything has gone - it better had!
    foreach my $key (keys %{$self})
    {
        # warn ref($self) . " still has '$key' key left after release.
+\n";
        $self->{$key}=undef;
        delete($self->{$key});
    }
}
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