The interpolation trick is:

$perl -de1
  DB<1> x $a  
0  undef
  DB<2> x $str = ' \$a is $a and \$a*2 is ${\($a*2)} '
0  ' \\$a is $a and \\$a*2 is ${\\($a*2)} '
  DB<3>  $a = 3

  DB<4>  x eval qq{"$str"}
0  ' $a is 3 and $a*2 is 6 '
  DB<5>  $a = 5

  DB<6>  x eval qq{"$str"}
0  ' $a is 5 and $a*2 is 10 '
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But think twice before sending the result to system()