Use

$string =~ tr/A-Za-z//s;
[download]

my $str = "foo baa bzz";
print $str, $/;
$str =~ s< ([A-Za-z]) (?:\1)+ >($1)gx;
print $str, $/;

__output__

foo baa bzz
fo ba bz
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_________
broquaint

$string =~ s/(.)\1+/$1/g;

# or...
#$string =~ tr/A-Za-z0-9//s;
$string =~ tr///sc;           # update: use this instead
[download]

Note that if you use tr///, you have to specify every character you want to match.

Update: Ah, true. Thanks, BrowserUk.

bbfu
Black flowers blossum
Fearless on my breath

If you want to remove contiguous duplicates of literally every character, you don't have to specify any characters with tr///s, you just add the /c option.

$s = 'Ttthe Qqquiicccck brroooown fox'
$s =~ tr///cs
print $s
Tthe Qquick brown fox
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Examine what is said, not who speaks.

The 7th Rule of perl club is -- pearl clubs are easily damaged. Use a diamond club instead.