As an addendum to turnstep's fine post: Another useful test is: ne ... as in:

if ($string1 ne $string2) {
    print "\$string1 is not equal to \$string2\n";
}
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Just as: == tests equality between two numbers,
!= tests inequality for numbers and
eq tests strings for equality as
ne tests strings for inequality.
I often find it useful to test

if ($string0 ne "") { #use $string0
}
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to verify that a string has a value even if it's == 0.