The Perldoc here says

Unary "+" has no effect whatsoever, even on strings. It is useful syntactically for separating a function name from a parenthesized expression that would otherwise be interpreted as the complete list of function arguments.

and to illustrate this I did this

use warnings;
use strict;
my $return=&call();
print "Return value :$return";

sub innercall($)
{
    print "I am called\n";
    return 'gzip';
}
sub call()
{
    return +(innercall('test')||'') =~ /gzip/;
}
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The call

return +(innercall('test')||'') =~ /gzip/;
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and

return (innercall('test')||'') =~ /gzip/;
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exhibit same behavior!Hope this helps!!

PS:Just in case you are not aware, the line

return +($r->header_in('Accept-Encoding')||'') =~ /gzip/;
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would return either a '1' or ' ' depending on the match success and failure respectively