Read each line:

while (<>) {
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prepend the stuff and write it out

   print "> ", $_;
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You can do that using command-line switches and stuff:

perl -i.bak -n -e "print '> ', $_"  file1.txt file2.txt etc.etc
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Look up -i -e -n in the perlrun man page. Get a copy of the Perl Cookbook, or one of the introductary texts.

—John