Basically, each time you say my $variable you get a brand, spanking new variable. So you're reading a number into the $number variable, but then you say my $number and get a brand, spanking new variable, which happens to have undef as it's value.

my $number;
$number = 40;
my $number;

if ($number > 39) {
  print "bigger than 39\n";
}
elsif ($number < 1) {
  print "smaller than 1\n";
}
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