my %valid_apt_numbers;
for my $unit ( split( /,/, q{1001,1002,...YADDAYADDA...,2505} ) ) {
  $valid_apt_numbers{ $unit } = 1;
}

if( exists $valid_apt_numbers{ $digits_4 } ) {
  $text =~ s{\b($digits_4)\b}{<a href=YADDAYADDA</a>}xmsg;
}