Without changing your algorithm in any way, you can gain a few characters by removing unnecessary quotes, and substituting $o with $_. Since say and length both default to $_ the change of the variable removes four extra characters. 
perl -E '@a=(0..9,A..Z,a..z);$_.=$a[rand@a]while 8>length;say'

[download]

And here's a completely non-golfed Perl 6 version that's even shorter: 

perl6 -e 'say (0..9, "A".."Z", "a".."z").pick(8, :repl)'
JIEFoUyiq

[download]

On my Linux box this is even shorter: 

$ pwgen -s 8 1
mUM6fAiz

[download]

But I guess not using Perl in a golfing challenge is cheating ;-)

In reply to Re: Golfing password creation by moritz
in thread Golfing password creation by kyle

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