The dereferenced arrayref technique works fine: "a string @{[foo()]}\n". But it's a little magical; it requires some tribal knowledge or testing for a reader of the code to understand what is happening.

Perl also provides printf, which is well documented and should be clearer for a reader:

sub foo { return "bar" }
printf "foo returns %s\n", foo();
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Additionally, there are a lot of template systems available to Perl. Template::Toolkit, Text::Template, Mojo::Template, Template::Tiny, and many others. Here is an example using Template::Toolkit:

#!/usr/bin/env perl

use strict;
use warnings;
use Template;

sub foo {return "bar"}

my $output;

Template
    ->new
    ->process(
        \"foo returns [% f %]\n",
        {f => foo()},
        \$output,
    );

print $output;
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This code works as-is with Template::Tiny if you just replace any mention of Template with Template::Tiny.