If you know the characters you want, just eliminate everything else.

#!perl
use strict;
use warnings;
use HTML::TableExtract;
use LWP::UserAgent ();

my $url = 'http://www.nasdaq.com/extended-trading/premarket-mostactive
+.aspx';
my $headers = ['Symbol', 'Last Sale*', 'Change Net / %', 'Share Volume
+'];

my $ua = LWP::UserAgent->new;
$ua->timeout(10);
$ua->env_proxy;

my $response = $ua->get($url);
if ( !$response->is_success) {
  die $response->status_line;
}
my $htm = $response->decoded_content;
  
# table4 
my $table_extract = HTML::TableExtract->new(
     count => 4, headers => $headers);  
my $tbl = $table_extract->parse($htm);
my $data = cleanup($tbl);
report('Advances',$data);

# table5 
$table_extract = HTML::TableExtract->new(
     count => 5, headers => $headers);  
$tbl = $table_extract->parse($htm);
$data = cleanup($tbl);
report('Decliners',$data);

sub cleanup {
  my $table = shift;
  my @data = ();
  for my $row ($table->rows) {
    my @clean = map{ s/[^A-Z0-9%,+-\.]/ /g; # allowable
                     s/^ +| +$//g; # trim spaces
                     $_ } @$row;
    push @data,\@clean;
  }
  return \@data;
}

sub report {
  my ($title,$data) = @_;
  print "$title\n";
  for (@$data){
    my ($stock,$openpr,$tmp,$vol) = @$_;
    my ($change,$pct) = split / +/,$tmp;
    my $closepr = $openpr - $change;
    print join "\t",($stock,'$'.$closepr,'$'.$openpr,$pct,$vol);
    print "\n";
  } 
}
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