Programming Perl, 4th Edition

Or Leave Out the Quotes Entirely

A name that has no other interpretation in the grammar will be treated as if it were a quoted string. These are known as barewords.^[50] As with filehandles and labels, a bareword that consists entirely of lowercase ASCII letters risks conflict with future reserved words. If you have warnings enabled, Perl will warn you about barewords. For example:

my @days = (Mon,Tue,Wed,Thu,Fri);
print STDOUT hello, " ", world, "\n";

sets the array @days to the short form of the weekdays and prints “hello world” followed by a newline on STDOUT. If you leave the filehandle out, Perl tries to interpret hello as a filehandle, resulting in a syntax error. Because this is so error-prone, some people may wish to avoid barewords entirely. The quoting operators listed earlier provide many convenient forms, including the qw// “quote words” construct, which nicely quotes a list of space-separated words:

my @days = qw(Mon Tue Wed Thu Fri);
print STDOUT "hello world\n";

You can go as far as to outlaw barewords entirely. If you say:

use strict "subs";

then any bareword will produce a compile-time error. The restriction lasts through the end of the enclosing scope. An inner scope may countermand this by saying:

no strict "subs";

Outlawing barewords is such a good idea that if you say

use v5.12;

or higher, Perl turns on all strictures for you automatically.

Note that the bare identifiers in constructs like:

"${verb}able"
$days{Feb}

are not considered barewords since they’re allowed by explicit ...