To not duplicate a pick you do a Fisher-Yates shuffle and stop after N steps:

my @ssn= getAllSSNs();
my $want= 1000;
for(  0..$want-1  ) {
    my $choice= $want + rand(@ssn-$want);
    @ssn[$_,$choice]= @ssn[$choice,$_];
}
return @ssn[0..$want-1];
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        - tye (but my friends call me "Tye")