$res->headers($header_file_content);

##</code><code>##

# in whatever package...

package HTTP::Response;

sub headers {
    my $obj = shift;
    my $arg = shift || die "no file nor string, aborted";
    my $fh;
    {
        no warnings 'io';  # stat might be unsuccessful
        open $fh, "<", -f $arg ? $arg: \$arg;
    }
    while( <$fh>) {
        chomp;
        my ($k,$v) = split/:\s+/,$_,2;
        $obj->header(lc $k, $v);
    }
}

package whatever; # back to whatever package