To stay closer to your original example, try separating the 'my' statement from the value assignment:

sub input
  {
      my $input;                   # Just the 'my' here
      while ( 1 ) 
      {
          $input = <STDIN>;        # Asssignment here
          chomp ( $input );

          if ( $input !~ /\d/ ) 
          {
              print "ILLEGAL INPUT\n";
              next;
          }
      
          last;
      }

      return $input;
  }
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Now the scope of '$input' is the entire sub block and 'return $input;' will work.