In Yiddish, "-n" is vocalic after a consonant. Vocalic -n is pronounced very similarly to -en (and in German it is even written -en). For example: In אַלטע זאַכן (alte zakhn), the -n is pronounced like a vowel, very similar to "-en". So therefore in Hebrew this is pronounced alte zachen (with a much clearer "e", but nevertheless the same syllable structure).
Same goes for -l. So words like ביסל (bisl), the -l is a vowel and is similar to -el, and so in Hebrew this would be pronounced bisel, but again with the same syllable structure. This goes for Hertzl as well.
The above two phenomena apply to Standard German as well, except that in Standard German, an "e" is usually written there.
What's different in Yiddish is after the letter "r".
In Standard German, words like Stern are pronounced as one syllable. In other words, the -n is not vocalic (instead the -r may be vocalic and form a diphthong). In Yiddish, however, it is pronounced as two syllables. The "r" is a pure consonant, even in dialects where it can vocalized, while the "n" is vocalic. So it is pronounced similar to shteren. And thus it is also pronounced shteren in Hebrew.
And same goes for the -rl ending as in perl. In Standard German, in words like this (e.g. geperlt), there is no extra syllable. In Yiddish, there is, and it transfers over to Hebrew.
If you want to compare, I recommend you look at how Hebrew borrows English words that end in -rn. Since in English, -rn does not create a second syllable, since the r is vocalic (while -rl in English does create a second syllable). So for example, do you say "cornflakes" as "kornfleyks" or as "korenfleyks"?