By surface analysis, *per- + *lьstь.
*perlьstь f
singular | dual | plural | |
---|---|---|---|
nominative | *perlьstь | *perlьsti | *perlьsti |
genitive | *perlьsti | *perlьstьju, *perlьsťu* | *perlьstьjь, *perlьsti* |
dative | *perlьsti | *perlьstьma | *perlьstьmъ |
accusative | *perlьstь | *perlьsti | *perlьsti |
instrumental | *perlьstьjǫ, *perlьsťǫ* | *perlьstьma | *perlьstьmi |
locative | *perlьsti | *perlьstьju, *perlьsťu* | *perlьstьxъ |
vocative | *perlьsti | *perlьsti | *perlьsti |
* The second form occurs in languages that contract early across /j/ (e.g. Czech), while the first form occurs in languages that do not (e.g. Russian).