You can fiddle with $COLUMS to change the table row-width

use strict;
use warnings;
use XML::Simple;
use Data::Dumper;

# read __DATA__ file into $_
{
    $/ = undef;
    $_ = <DATA>;
    close DATA;
}

# use XML::Simple to read the table string
my $ref = XMLin( $_ , NoAttr=>1 );
my $tr = $ref->{tr};

my @ARR;
# flatten table structure into an array
for my $row (keys @{$tr}){
    my @X = values @{(values %{ $tr->[$row] })[0]};
    push @ARR, @X;
}

# now we reformat the table
my $COLUMNS = 2;
my $counter = 0;
my %S;

for my $v (@ARR){
  my $i=int($counter/$COLUMNS);
  my $j=int($counter % $COLUMNS);
  push @{$S{'tr'}->[$i]->{'td'}}, $v;
  ++$counter;
}

# We use XML::Simple to transform this stucture to an xml/html string
my $xml = new XML::Simple (NoAttr=>1, RootName=>'table');
# convert Perl array ref into XML document 
my $new_table = $xml->XMLout(\%S);

# now show the result
print $new_table;


__DATA__
<table style="width:100%">
  <tr>
    <th>Firstname</th>
    <th>Lastname</th>
    <th>Age</th>
  </tr>
  <tr>
    <td>Jill</td>
    <td>Smith</td>
    <td>50</td>
  </tr>
  <tr>
    <td>Eve</td>
    <td>Jackson</td>
    <td>94</td>
  </tr>
</table>
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