My submissions:

Hole 1 (16 characters)

sub f{
(pop=~/w+/g)[4]
}
[download]

Hole 2 (31 characters) perl -e# -e# -e# -e# -egoto+TPC Hole 3 (46 characters)
If only alphanumeric data will be processed:

sub f{
$p=$_=pop;1while(!/^($p){2,}$/&&$p=~s/.$//);$p
}
[download]

If required to support non-alpha, or unusual characters in the input data (requires \Q...\E), then 51 characters:

sub f{
$p=$_=pop;1while(!/^(\Q$p\E){2,}$/&&$p=~s/.$//s);$p
}
[download]

Hole 4 (24 characters)

sub f{
map{chr hex$_}pop=~/../g
}
[download]

Hole 5 (30 characters)

sub f{
$x=pop;map{$_>$x?$x=$_:0}@_;$x
}
[download]

Hole 6 (39 characters)

sub f{
grep{s/.//}sort map{/[aeiou]/;"$&$_"}@_
}
[download]

Update:
Corrected hole 4 (returned string, wanted list).
Reduced hole 6 (@x=@_ to @_, used $& instead of memorizing)
Removed first 'version' of hole #5, which didn't handle negative numbers, leaving the longer second version.

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