perl -p -i.bak -w -e 's/dude/Johnny5/g' myinfo.txt

The most confusing part of this is probably all those switches. Take a look at perlrun for help (the following instructions are directly from there).

The -p switch assumes the following loop around your program.

LINE:
    while (<>) {
    ...        # your program goes here
    } continue {
    print or die "-p destination: $!\n";
    }
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As you can see, it looks like the body of your perl program.

The -i switch specifies that files processed by the <> construct are to be edited in-place. It does this by renaming the input file, opening the output file by the original name, and selecting that output file as the default for print() statements. The extension, if supplied, is used to modify the name of the old file to make a backup copy.

You can see that in the one-liner, the new file's filename will be appended with '.bak'.

The -w switch is to turn on warnings. Simple enough.

The -e switch tells Perl to run any following code (which is between the apostrophes).

The substitution in the actual code is simply globally replacing all instances of 'dude' with 'Johnny5'. It is more vague than your regex, and it will replace all instances, not just the first one.

Finally, the filename at the end of the one-liner is the argument to the code.

In other words, that one liner can be rewritten in a file as:

#!/usr/bin/perl -w

$extension = '.bak';

LINE:
    while (<>) {
        if ($ARGV ne $oldargv) {
            if ($extension !~ /\*/) {
                $backup = $ARGV . $extension;
            } else {
                ($backup = $extension) =~ s/\*/$ARGV/g;
            }

            rename($ARGV, $backup);
            open(ARGVOUT, ">$ARGV");
            select(ARGVOUT);
            $oldargv = $ARGV;
        }

    s/dude/Johnny5/g;
    } continue {
        print or die "-p destination: $!\n";
    }
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Pretty cool, eh.