$file0 is not blank. It contains \n. Just use chomp to remove the newline before checking if the variable is "empty".

my $file = <STDIN>;
chomp($file);
$file ||= "somelist";
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The above will also use the default if 0 (zero) is entered. The following avoids that problem:

my $file = <STDIN>;
chomp($file);
$file = "somelist"
   if not length $file;
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By the way, you really, really, really should use use strict; and use warnings;.