Is the string empty?

if ($str eq '') {
    print "String is empty.";
}

That would work, but if use warnings; is in effect, as it should be, then you might get a Use of uninitialized value warnings if $str is undef. So it might be better to check this too, before any other comparison:

if (not defined $str) {
    print "String is so empty, it is not even defined.";
}

and to do the other comparisons only if $str is defined.

Has the string only spaces in it?

if ($str =~ /^ *$/) {
    print "String contains 0 or more spaces and nothing else.";
}

Has the string only white-spaces in it?

if ($str =~ /^\s*$/) {
    print "String contains 0 or more white-space character and nothing else.";
}

A white-space character can be a space, a tab, and a few other characters that normally we cannot see. The >^ at the beginning of the regex means "match at the beginning of the string". The $ at the end of the regex means "match at the end of the string".

* in the regex is a quantifier. It means match 0 or more times the thing that is on its left hand side. In the previous regex there was a space in on the left hand side of the *. In the second regex we have \s in front of the *

\s matches any on of the following 5 characters:

• space
• tab (\t)
• carriage-return (\r)
• newline (\n)
• form-feed (\f