Programming Perl, 4th Edition

strict “subs”

This stricture makes Perl treat all barewords as syntax errors. A bareword (“bearword” in ursine dialects) is any bare name or identifier that has no other interpretation forced by context. (Context is often forced by a nearby keyword or token, or by predeclaration of the word in question.) Historically, barewords were interpreted as unquoted strings. This stricture outlaws that interpretation. If you mean to use it as a string, quote it. If you mean to use it as a function call, predeclare it or use parentheses.

As a particular case of forced context, remember that a bareword that appears by itself in curly braces or on the lefthand side of the => operator counts as being quoted, and so is not subject to this restriction.

use strict "subs";

$x = whatever;      # WRONG: bareword error!
$x = whatever();    # This always works, though.

sub whatever;       # Predeclare function.
$x = whatever;      # Now it's ok.

# These uses are permitted, because the => quotes:
%hash = (red => 1, blue => 2, green => 3);

$rednum =  $hash{red};              # Ok, braces quote here

# But not this one:
@coolnums = @hash{blue, green};     # WRONG: bareword error
@coolnums = @hash{"blue", "green"}; # Ok, words now quoted
@coolnums = @hash{qw/blue green/};  # Likewise