You have rnahi's answer - it's a shortcut, no offence but I think it's a bit too magic for your level of perl mastery. Here are my instructions:

• Learn how to open a file handle -  perldoc -f open
• Learn how to read the file content -  perldoc perlop look for the <> operator.
• Learn Perl regular expressions -  perldoc perlre
• Learn the substitution operator  perldoc perlop look for  s/PATTERN/REPLACEMENT/egimosx
• Learn how to write to a file -  perldoc -f print

Then the progarm should be obvious.