my ($itself) = grep { 0 <= index $0, $_ } $checker->dir->read;
See $0, index, grep. In other words, the read method returns the contents of the directory, grep then tries to find a file name $_ that's contained in the test script's path (it's buggy, as it doesn't check whether the preceding character is a slash).
m{(?:^|/)\Q$_\E$}x
The regex fixes the issue: it checks that the filename is present in the script's name from the beginning (^) or a slash to the end ($). \Q quotes special characters in the filename so it matches literally.
($q=q:Sq=~/;[c](.)(.)/;chr(-||-|5+lengthSq)`"S|oS2"`map{chr |+ord
}map{substrSq`S_+|`|}3E|-|`7**2-3:)=~y+S|`+$1,++print+eval$q,q,a,
|