That explanation isn't accurate. Perl complains about using \1 in the right-hand side of a substitution because using $1 is preferred. Whether the script is entered on the command line or not isn't important.

#!perl -w
use diagnostics;

$_ = 'foo';
s/(.)/\1/;
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This script produces the following output:

\1 better written as $1 at tmp.pl line 5 (#1)
 
    (W) Outside of patterns, backreferences live on as variables.  The
+ use
    of backslashes is grandfathered on the right-hand side of a
    substitution, but stylistically it's better to use the variable fo
+rm
    because other Perl programmers will expect it, and it works better
    if there are more than 9 backreferences.
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