A hash slice variation:

$H{$_}++ for split '', $str;
if (grep $_, @H{a..z,A..Z}) { 
  ...
}
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Or an array slice variation:

$A[ord $_]++ for split '', $str;
if (grep $_, @A[map ord, a..z,A..Z]) {
  ...
}
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Update: or forget the slices (duh):

$H{$_}++ for split '', $str;
if (grep $H{$_}, a..z, A..Z) { 
  ...
}
[download]

$A[ord $_]++ for split '', $str;
if (grep $A[ord $_], a..z, A..Z) {
  ...
}
[download]

Update #2: using POSIX:

use POSIX;
if ( grep { isalpha $_ } split '', $str ) {
  ...
}
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Add salt, quotes, and strict to taste.