Consider this:

my @ary = ('a','b','c','d');
print @ary;      # prints "abcd" as four 1-char string values
print "\n";
print "@ary";    # prints "a b c d" as a single string value
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An array in double-quotes interpolates with the array values separated by the value of the special variable: $" (which is space by default).

So the line you cite creates a single string consisting of all the concatenated elements of @_ separated by spaces.

HTH, David