if ( $word =~ /^([a-zA-Z]+)/ ) {
  $word = $1;
}
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the ^ matches the start of the string
the () assigns whatever it matches between them to $1

update the above matches any of [a-zA-Z] at the start of the string
If you want to match any character before the first : then do

if ( $word =~ /^([^:]+)/ ) {
  $word = $1;
}
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the [^:] means match anything except a :