This is not a general solution for interpolating subroutines. Whether you use "${\foo()}" or "@{[foo()]}", foo() will be running in list context. In contrast, foo() . '...' and "$username" both run in scalar context.

If the return value is context sensitive, the list context value will be returned instead of the expected scalar context value, as this code snippet demonstrates:

sub foo {
  wantarray() ? 10 : 1;
}

print 'Scalar context: ';
print ''. foo();
print ', ';
print foo() . "\n";

print 'List context: ';
print "@{[foo()]}, ";
print "${\foo()}\n";

# outputs 
Scalar context: 1, 1
List context: 10, 10
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To preserve the scalar context without stuffing the return value into a variable, one must use concatentation, e.g. '' . foo() or  "mumble " . foo() . " the burrowgroves" (or forced scalar context as ig points out below).

Best, beth

Or force a scalar context on the interpolated subroutine calls:

sub foo {
  wantarray() ? 10 : 1;
}

print 'Scalar context: ';
print ''. foo();
print ', ';
print foo() . "\n";

print 'List context: ';
print "@{[foo()]}, ";
print "${\foo()}\n";

print 'Scalar context: ';
print "@{[scalar(foo())]}, ";
print "${\scalar(foo())}\n";
__END__
Produces:
Scalar context: 1, 1
List context: 10, 10
Scalar context: 1, 1
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But, while this is possible, I can't imagine it being preferable to concatenation for producing a scalar context.

So complicated and ugly!

That's true, of course... but some people just want to interpolate :) — I have to admit I've also felt that urge at times...

This is probably the outcome of the generalizing behavior of our brains: we've learnt we can interpolate scalars and arrays, so why not also hashes, functions, anything?  (This behavior, in and of itself, is certainly a Good Thing, BTW, otherwise we probably wouldn't be where we are today, messing with Perl... supported by those little silicon chips, etc.)

Also, some people just find

    "abc ${\config('username')} xyz";
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easier to read/type than

    "abc ".config('username')." xyz";
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even though they are exactly the same number of chars.

Anyhow, the nice thing is that with Perl's TIMTOWTDI-ness, everyone can get happy :)

It is in a print, so a comma would be more natural than a dot. There is no need to build a single scalar when the operator takes a list.

print config('username'), " is my username";
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Most likely the OP would want a \n in there too, but that isn't really part of the interpolation question.