The "short hand" is a Perl one liner designed to be run from the command line. The -i.bak puts an in place edit loop around the code provided with -e and does the same thing as $^I = "myinfo.txt.bak"; is intended to do (but doesn't, see below for a correct version).

The diamond operator uses a file handle or, if none is provided, opens file handles for each file name in @ARGV (generally those provided on the command line). Your code mixes the two distinct usages. Consider instead:

use strict;
use warnings;

my $filename = 'myinfo.txt';

# Create the test file
open my $outFile, '>', $filename or die "Failed to create $filename: $
+!";
print $outFile <<FILE;
Program name: some junk
Author: dude
Company: no name
Phone: 123.345.1234
Version: 1.0
FILE
close $outFile;

# 'Edit' the file
local @ARGV = $filename;
local $^I = '.bak';

while (<>) {
    next if /^Phone:/;
    
    s/^Author:.*/Author: Johny5/;
    print;
}
[download]

which updates myinfo.txt to contain:

Program name: some junk
Author: Johny5
Company: no name
Version: 1.0
[download]

and generates myinfo.txt.bak with the original contents of myinfo.txt.