It may be surprising at first, but the idea is that it lets you construct regexps on the fly. One common thing this is used for is when you have a list of valid values you got from somewhere, say @valid, and you want to check a value against it:

my $valid = join "|", @valid;
print "okay" if /^$valid$/;
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There are actually two improvements to make in the above code. First, the members of the valid list themselves might contain metacharacters in need of quoting; second, Perl has the qr// operator to make this more efficient:

# don't run this code on every match: the idea is the qr// needs
# to be computed only once.
my $valid    = join "|", map { quotemeta } @valid;
my $valid_re = qr/^$valid$/;

# now match as many times as you like.
print "$_: " . (/$valid_re/ ? "okay" : "not okay") . "\n"
        for @a_bunch_of_inputs;
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