You forgot a bunch of commas.

Golfing:

$new{$_}=defined$old{$_}?$old{$_}:999 for@mips
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If there are no 0 values:

$new{$_}=$old{$_}||999 for@mips
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The first, but perl6 (I think):

$new{$_}=$old{$_}//999 for@mips
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The first one written in a more readable way:

for (@mips) {
    $new{$_} = defined($old{$_}) ? $old{$_} : 999;
}
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Another approach:

%new=map{($_,defined$old{$_}?$old{$_}:999}@mips
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If 0 may be 999:

%new=map{($_,$old{$_}||999)}@mips
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Another perl6 try:

%new=map{($_,$old{$_}//999)}@mips
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Again, the first of this approach written in a readable manner:

%new = map {
    ($_ => defined($old{$_}) ? $old{$_} : 999)
} @mips;
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Good luck.

2;0 juerd@ouranos:~$ perl -e'undef christmas'
Segmentation fault
2;139 juerd@ouranos:~$
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