A bug in musl and/or perl (where the bug is is a matter of debate) is causing an issue in some OpenWrt builds. In affected builds, the perl string search functions, like index(), are broken.
For example:
#!/usr/bin/perl
my $string1 .= "abcabc";
my $string2 .= $string1;
printf "string1='%s', string2='%s', index=%s\n", $string1, $string2, index($string1, $string2);
On affected builds, you will see index=-<largenum>. On non-affected builds, you will see index=0.
This issue seems to affect all aarch64 builds. It doesn't seem to affect 32 bit arm builds, which probably means there are more overall devices not affected. I don't yet know if it affects mips or x86.
I suspect likely no on the former and yes on the latter.
The issue can be worked around by defining _GNU_SOURCE when perl is built (requires patching OpenWrt). The problem was already corrected by the perl devs for perl >= 5.30, so the issue will go away if we get a new version of perl.
There aren't a whole lot of perl programs that use index() or related functions. But those that do will fail or bug in odd ways.