Thanks for your support. I tried

perl -e '$a = "foobar"; printf("#%16s#\n", $a);'
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and got

#          foobar#
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as was to be expected - that was the reason I posted my question here. Even using static strings as you did in your example are working:

Two names: 'foo             ', 'bar             ' for         D#/Eb4, 
+F#/Gb4, A#/Bb4, F5,
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It must be something with the two variables, but I have no clue...