It would make for good hygiene/habits to initialize that way, but it's not mandatory (and probably won't make a difference in this case); the following works with no sweat:

use strict;
use PDL;

my $arref;         
while(<DATA>) {
  push @$arref, $_;
}
my $p = pdl($arref);
print $p,$/;

__DATA__
1.0
2.2
3.1
4.0
5.5
10.0
20.0
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