After some rooting around in the Word VBA docs and studying Using Win32::OLE and Excel - Tips and Tricks I came up with this.

(needs Word to be open with a document open containing a table)

#!/usr/bin/perl

use strict;
use warnings;
use Win32::OLE;

my $w = Win32::OLE->GetActiveObject('Word.Application');

my $d = $w->ActiveDocument;
my $table = $d->Tables(1);

my $first_cell  = $table->Cell({Row => 1, Column => 1});
my $second_cell = $table->Cell({Row => 1, Column => 2});
$first_cell->merge({MergeTo => $second_cell});

$d->save(1);
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