This works:

perl -pi -e 'BEGIN{undef$/};s/(.*)/<$1>/s' filename.txt

A couple of notes:

• Use $1 instead of \1 (use \1 only in the left part of the substitution
• The /g switch fell off (you only wanted to do it once)

And... I guess that's it... I undefined $/ so that the file would be read all at once :-)

HTH