This has been discussed numerous times before:

• Perl4Win32: prettify code syntax
• Feature suggestion for code-tags
• Code Highlighting in the code tag
• Feature request: Colourful comments?
• syntax highlighting of code in perlmonks forums
• Vim for PerlMonks nodes
• HTML: Syntax highlightning for perl-Code
• On-the-fly all-languages syntax highlighting
• Feature Request: Adding Colors to Source Code
• (Client side) Perl Syntax Highlighter
• Code highlighting?
• Free Nodelet hack: Syntax colouring
• Use your Vim colorscheme in perlmonks syntax highlighting
• Syntax highlighting code tags
• PM Code Tidier/Highlighter

See also:

• Code2HTML
• Gtk2::SourceView
• PPI::HTML

Update: This is now a FAQ.