Syntax And Variables
The simplest Perl variables are “scalar” variables which hold a single string or number. Scalar variable names begin with a dollar sign ($) such as $sum or $greeting. Scalar and other variables do not need to be pre-declared — using a variable automatically declares it as a global variable. Variable names and other identifiers are composed of letters, digits, and underscores (_) and are case sensitive.
Comments begin with a “#” and extend to the end of the line. $x = 2; ## scalar var $x set to the number 2
$greeting = “hello”; ## scalar var $greeting set to the string “hello”
A variable that has not been given a value has the special value “undef” which can be detected using the “defined” operator. Undef looks like 0 when used as a number, or the empty string “” when used as a string, although a well written program probably should not depend on undef in that way. When Perl is run with “warnings” enabled (the -w flag), using an undef variable prints a warning.
if (!defined($binky)) {
print “the variable ‘binky’ has not been given a value!\n”;
}
What’s With This ‘$’ Stuff?
Larry Wall, Perl’s creator, has a background in linguistics which explains a few things about Perl. I saw a Larry Wall talk where he gave a sort of explanation for the ‘$’ syntax in Perl: In human languages, it’s intuitive for each part of speech to have its own sound pattern. So for example, a baby might learn that English nouns end in “-y” — “mommy,” “daddy,” “doggy”. (It’s natural for a baby to over generalize the “rule” to get made up words like “bikey” and “blanky”.) In some small way, Perl tries to capture the different signature-for-different-role pattern in its syntax — all scalar expressions look alike since they all start with ‘$’.
