my $newfont = '';
# maybe font value read from user, results in
# $newfont = "{liberation sans} 8 normal";
my $opt = "-fg => $fg, -bg => $bg";
$opt .= ", -font => $newfont" if length($newfont);
$rw->Label(-text => "Col. 1", $opt, -padx => 10)->grid(
   $rw->Label(-text => "Col. 2", $opt, -padx => 11),
   $rw->Label(-text => "Col. 3", $opt, -padx => 11))

##</code><code>##

Tk::Error: Odd number of args to Tk::Label->new(...)