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open (INFILE, "input.txt") or die "cannot open input: $!";
open (OUTFILE, ">output.txt") or die "cannot open output: $!";
...

close INFILE;
close OUTFILE;

perldoc -f open
perldoc perlvar (look for the $/ entry)
perldoc perlre (for info about regular expressions)

-- 
Joost       downtime n. The period during which a system
            is error-free and immune from user input.