Part 1 with extra credit (since that is the only way it should be done) and matching case insensitive 
sub f {
($a=shift)=~s/_/[^$a]/g;grep/^$a$/i,@{$_[0]}
#23456789_123456789_123456789_123456789_1234
}

[download]
44 chars

Update I screwed up and didn't do most frequent char... now I have both... 

sub g {
shift;for(@{+shift}){s/[@_]//g;$c{$_}++for split//}(sort{$c{$b}<=>$c{$
+a}}%c)[0]
#23456789_123456789_123456789_123456789_123456789_123456789_123456789_
+123456789
}

[download]
Done the right way I get 79 chars for part B
sub g {
shift;for(@{+shift}){s/[@_]//g;$b||=chop}$b
#23456789_123456789_123456789_123456789_123
}

[download]
43 chars without most common char (the wrong way :)

                - Ant
                - Some of my best work - Fish Dinner

In reply to Re: (Golf) Hangman Player by suaveant
in thread (Golf) Hangman Player by Masem

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