Well, if you read the documentation, it says about getpwent:

These routines are the same as their counterparts in the system C library.

If this does not help, google it: getpwent, first find is http://man7.org/linux/man-pages/man3/getpwent.3.html and says:

The first time getpwent() is called, it returns the first entry; thereafter, it returns successive entries.

This little piece of code shows what happens:

while(@x=getpwent()){
    print "@x\n";
}
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