separate limiter values with a pipe (|)

It can be easily done by CSS.

I always prefer CSS for this kind of tweaks.

.limiter .control a + a::before {
   content: " | ";
}

It will Add a Pipe separator between a tag

Demo can be found at Fiddle

Hope above will help!

Try using PHP implode(), check below modified code:

<div class="field limiter">
    <label class="label" for="limiter">
        <span><?= /* @escapeNotVerified */ __('View:') ?></span>
    </label>
    <div class="control">
        <?php foreach ($block->getAvailableLimit() as $_key => $_limit): ?>
            <a data-role="limiter" href="#" data-value="<?php /* @escapeNotVerified */ echo $_key ?>"<?php if ($block->isLimitCurrent($_key)): ?>
                class="selected"<?php endif ?>>
                <?php /* @escapeNotVerified */ echo implode("|",$_limit) ?>
            </a>
        <?php endforeach; ?>
    </div>
</div>