if ($file =~ /^\d{8}(?:_\d+)*\.ilf$/i ||
    $file =~ /^\d{8}(?:\s\d+)*\.sht$/i
) {
   ...
}

##</code><code>##

      vvvvvvvvv  vvv vvv   vvvv  common
     /^\d{8}(?:_ \d+)*\.ilf$/ix
     /^\d{8}(?:\s\d+)*\.sht$/ix
               ^^       ^^^      not common

##</code><code>##

            (?:_ \d+)*\.ilf
     /^\d{8}               $/ix
            (?:\s\d+)*\.sht

##</code><code>##

if ($file =~ /^\d{8}(?:(?:_\d+)*\.ilf|(?:\s\d+)*\.sht)$/i) {
   ...
}