#!perl
use strict; # <--- add

open(my $fh, '<', 'Citytempfile.txt')
 or die "Cannot open Citytempfile.txt: $!";
#                     ^^^^ corrected

my $i = 0;
#        ^ missing ;

while (<$fh>) {
#my @array = split ' ', <$fh>
  my @array = split ' ', $_; # change
                      
  print $i+1, '. Temperature in ',$array[0], 
      ' is ', $array[1], 
      ' but average temp in ', $array[2], 
      ' is ', $array[3]," degrees.\n" ;
  $i++;
}