What you want to do is called inplace editing. Here is an example:

#!/usr/bin/perl -i.bak
{
    local $/ = "//\n";
    while (<>) {        # grab a line into $_
        s/this/that/;   # munge away
    print;          # output the munged line to file
    }
}
continue {
    print STDOUT ++$counter, "\n";
}
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The -i.bak tells perl to do an inplace edit on the argument files and write a backup called <file>.bak where <file> is the argument file

You set the input separator as shown - it is localised to the block. The while than loops though each "line" assigning it to $_. We can make mods to $_ and then the print prints it back to the original file overwriting the original line. The contents of the original file (unmodified) are written to <file>.bak. We can print to STDOUT by specifying it and use a continue block to do something (like print the counter) with each line.

Just call the script like this: perl munge.pl <file1> <file2>

cheers

tachyon

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