The concatenation operator (dot), described in perlop, is not intended to be used inside a string. It is expected to join strings. So the following work:

"foo" . '_' . 123
$abc . '_' . 123
"foo" . '_' . $num
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But the following do not work, because dot inside a string is just a dot, not an operator:

"$abc . _ . 123"
"foo . _ . $num"
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What you want is one of the following:

"${abc}_$_"            # 1
$abc . '_' . $_        # 2
join('_', $abc, $_)    # 3
"$abc\_.$_"            # 4
$abc . "_$_"           # 5
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I prefer option 1, 2 or 3. Option 2 is probably the most legible.

Perl would probably make a lot more sense if you read perlintro, perlsyn, perlop, perlsub, and perldata. That's a couple hours worth of material, but it would move you forward in your learning curve.

In perlop you can also read The Gory Details of Parsing Quote and Quote-like Constructs.