This is because text returned by perldoc is no more POD, but is just plain text. For example:

$ head /usr/share/perl/5.14/pod/perldoc.pod

=head1 NAME

perldoc - Look up Perl documentation in Pod format.

=head1 SYNOPSIS

B<perldoc> [B<-h>] [B<-D>] [B<-t>] [B<-u>] [B<-m>] [B<-l>] [B<-F>]
[B<-i>] [B<-V>] [B<-T>] [B<-r>]
[B<-dI<destination_file>>]
$ perldoc perldoc | head
PERLDOC(1)            User Contributed Perl Documentation           PE
+RLDOC(1)



NAME
       perldoc - Look up Perl documentation in Pod format.

SYNOPSIS
       perldoc [-h] [-D] [-t] [-u] [-m] [-l] [-F] [-i] [-V] [-T] [-r]
       [-ddestination_file] [-oformatname] [-MFormatterClassName]
[download]

Try perldoc -u <whatever> | pod2pdf instead (option -u makes perldoc show just raw Pod source).