Answer

In Perl, interpolation (variable substitution) is supported in double quotes ("") and certain templates (e.g., heredoc). Scalars are interpolated simply:

my $name = 'Bob';
print "Hello, $name!
"; # Hello, Bob!

Arrays in double quotes become a string after substitution, with elements separated by the character $" (by default — a space):

my @items = ('a','b','c');
print "List: @items
"; # List: a b c

Hashes are substituted as a key=>value string, separated by the character $;:

my %h = (x => 1, y => 2);
print "Hash: %h
"; # Hash: x1y2 (may vary)

In single quotes (' ') nothing is interpolated — the string is output literally.

It is also possible to use complex expressions for interpolation through curly braces:

print "${name}_test
"; # Bob_test

The danger lies in accidental interpolation of special characters and the differing behavior of arrays, hashes, and scalars.

Trick Question

Why does the string "Total: $total$val" sometimes work incorrectly?

Common answer: "Everything is substituted as it is!"

Correct answer:

The string "Total: $total$val" can only correctly substitute the variable $total, the variable $val may not be recognized or Perl may interpolate the wrong range if variable names are similar. To avoid ambiguity, use curly braces:

print "Total: ${total}${val}
";

History

History 1

In the reports, the total string looked like this: "Total: $sum rub." After adding the variable $rub to the code, the string became "Total: $sum$rub", resulting in interpolation working only for $sum$, while $rub was skipped, leading to warnings. No one understood until they saw an empty string in the output.

History 2

When forming an SQL query, array interpolation through @values was used without join: "IN (@values)". This resulted in a string with a space separator instead of the expected comma-separated list. The outcome — an SQL error and incorrect selection.

History 3

To output the list of hash keys, %h was used in the string: "Keys: %h". As a result, a concatenated key-value string was produced instead of a nice list of keys. It should have been: join ", ", keys %h.

Explain how array, hash, and scalar interpolation works in Perl for different types of strings, and what dangers are associated with this mechanism.

Answer

Trick Question