$text='Learning Perl? http://www.perlmonks.org/ <this> is the place';

  $text =~ s!(http://\S+)
                      (\s+<([^>]+)>)?
                     !
                      defined($3)?
                        qq(<a href="$1">$3</a>):
                        qq(<a href="$1">$1</a>)
                     !gex;
print $text;