You've created an unnecessarily complex example to replicate your observation - you get the same error with: 

use strict; 
(my $x = 15) && $x;

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and obviously the following does not throw the error: 

use strict; 
(my $x = 15);$x;

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Unfortunately I don't have a good why scoping behaves the way it does in these examples - I would naively expect the my statement to execute before the second term in the conditional was evaluated. I found the documentation you want to read. From Private Variables via my() in perlsub:

The declared variable is not introduced (is not visible) until after the current statement.

So there you go. I assume the design logic is so the following works-as-hoped: 

my $x = 1;
for (1 .. 5) {
    my $x = $x;
    $x += $_;
    print $x;
}
print $x;

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In reply to Re: Declaring a lexical within a 'if' condition by kennethk
in thread Declaring a lexical within a 'if' condition by rovf

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