http://perldoc.perl.org/perlintro.html#Scalars

print "The square of $answer is ", $answer * $answer, "\n";
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print takes a list :) this list can be of a single item. join takes a list and makes it a single string

print join '', "The square of $answer is ", $answer * $answer, "\n";
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you can also use the join operator (concatenation)

print "The square of $answer is ". $answer * $answer. "\n";
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If you insist on interpolation, see

http://perldoc.perl.org/perltrap.html#Interpolation-Traps

@ always interpolates an array in double-quotish strings

Arbitrary expressions are evaluated inside braces within double quotes