It's the same trick as in

while (my $line = <$fh>) {
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which is a shorthand for

while (defined( my $line = readline($fh) )) {
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where, again, my $line appears as an argument operand of = which in turn is an argument operand of defined. my has two effects, one compile time and one run time. In compile time, you declare a lexical variable, and in runtime, you use it as an lvalue (i.e. assign a value to it). Variables declared in block conditions (if, while, unless, until) are accessible until the last conditional operator block.

if (my $x = func()) {
    print "$x accesible here";
} elsif ($x eq 0) { # even here,
   print "and also here: $x";
}
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Updated.