I'm reading HTTP::headers code. I understand what GET mean, when only one argument, @_ becomes empty and $op will choose GET. But I can't understand PUSH and SET logic before the 2nd "?". Before receiving any value, $seen{field_name} must be empty, so what does $seen{field_name}++ mean? What's difference of PUSH and SET?

Another question is if wantarray line. I read http://perldoc.perl.org/functions/wantarray.html but didn't understand."if the context of the currently executing subroutine or eval is looking for a list value". What "the context" refers here, @old returned by $self->_header? 

sub header
{
    my $self = shift;
    Carp::croak('Usage: $h->header($field, ...)') unless @_;
    my(@old);
    my %seen;
    while (@_) {
    my $field = shift;
        my $op = @_ ? ($seen{lc($field)}++ ? 'PUSH' : 'SET') : 'GET';
    @old = $self->_header($field, shift, $op);
    }
    return @old if wantarray;
    return $old[0] if @old <= 1;
    join(", ", @old);
}

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I delete the line about "++" to state more clear about my question.

In reply to What's this line means in HTTP::headers? by anaconda_wly

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