you could play with CGI.pm's innards to get a stack trace when font is called...

use CGI::Carp qw[ confess ];
my $old_font = \&CGI::font;
*CGI::font = sub { confess "font here!"; $old_font(@_) };
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or you can just replace it with a sub that returns a "<span class=""> with the class determined by what's passed in...

(udpate: this does involve you editing the code manually though)