You can't concatenate onto a readonly scalar, in either Perl or XS. 
#!/usr/bin/perl
use strict;
use warnings;

use Inline C => <<'END_C';

void 
readonly(SV *sv) {
    SvREADONLY_on(sv);
}

END_C

my $foo = "foo";
readonly($foo);

# This dies with: 
# "Modification of a read-only value attempted at cat_readonly.plx lin
+e 16."
$foo .= "bar";

# unreachable:
print "$foo\n";

[download]

So, if you have to deal with the possibility that your first scalar might be readonly, you have to do $a = $a . $b or its equivalent in XS rather than $a .= $b. It might be wasteful, but it's the only way.

--
Marvin Humphrey
Rectangular Research ― http://www.rectangular.com

In reply to Re^5: XS/Inline::C concat *any* two SVs. by creamygoodness
in thread XS/Inline::C concat *any* two SVs. by BrowserUk

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