In unix, creating a string in double quotes will cause interpolation of whatever the shell thinks are shell variables, e.g.

 perl -we "$b=14; print $b"
syntax error at -e line 1, near "="
Execution of -e aborted due to compilation errors.
[download]

or:

echo hello world
perl -we "if($_=~/World/){print 111}"

Unquoted string "world" may clash with future reserved word at -e line
+ 1.
111
[download]

Although your posted one-liner causes no complains in my linux/bash, maybe is not working as expected since $_ has a special meaning in bash (and possibly other shells): it is the last parameter of the last command (more special variables here: http://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_05_02)

So, Unquoted string "world" is caused by the interpolation of $_ by bash shell

bw, bliako

Trick #8: $ENV{}

    When you're writing a one-liner using -e in the shell, you generally want to quote it with ', so that dollar signs inside the one-liner aren't expanded by the shell. But that makes it annoying to use a ' inside your one-liner, since you can't escape a single quote inside of single quotes, in the shell.

    Let's suppose we wanted to print the username of anyone in /etc/passwd whose name included an apostrophe. One option would be to use a standard shell-quoting trick to include the ':

    perl -F: -lane 'print $F[0] if $F4 =~ /'"'"'/' /etc/passwd

    But counting apostrophes and backslashes gets old fast. A better option, in my opinion, is to use the environment to pass the regex into perl, which lets you dodge a layer of parsing entirely:

    env re="'" perl -F: -lane 'print $F[0] if $F4 =~ /$ENV{re}/' /etc/passwd

    We use the env command to place the regex in a variable called re, which we can then refer to from the perl script through the %ENV hash. This way is slightly longer, but I find the savings in counting backslashes or quotes to be worth it, especially if you need to end up embedding strings with more than a single metacharacter.

from https://blogs.oracle.com/linux/the-top-10-tricks-of-perl-one-liners-v2

Or just use escapes, or if it's about passing something from the shell to Perl, the -s option...

$ perl -sle 'print qq{\047\x27$x}' -- -x="'"
'''
$ FOO='`´()/~?*+#"@$%&'"'"; perl -sle 'print $x' -- -x="$FOO"
`´()/~?*+#"@$%&'
[download]