Your index() solution does not do exactly what the regex solution does.

This shortcoming can be addressed by the use of rindex:

Win8 Strawberry 5.8.9.5 (32)  Mon 06/06/2022 20:19:26
C:\@Work\Perl\monks
>perl

use strict;
use warnings;

my $path = "/some/3.5.2+tcl/path/3.5.2+tcl/a/b/c";
print "\$path before: '$path' \n";

my $version = "3.5.2+tcl";

my $pos = rindex($path, $version);
# cut everything beyond the match, if there was a match
substr($path, $pos + length($version)) = '' if $pos >= 0;
print "\$path after:  '$path' \n";

^Z
$path before: '/some/3.5.2+tcl/path/3.5.2+tcl/a/b/c'
$path after:  '/some/3.5.2+tcl/path/3.5.2+tcl'
[download]

Personally, I have no objection to \K on the grounds of understandability, readability or maintainability; indeed, it seems desirable on these grounds. One must be aware, however, that it was only introduced with Perl version 5.10. That's over twenty years old now, but one still occasionally sees situations in which \K is not available. AFAIK, your regex approach and the rindex approach will work with any version of Perl 5.