... must contain a mix of both letters and numbers.
... good words are the words that are a mix of letters and numbers.

The specification and example in the OP is a bit unclear to me, but, taken with some of the other replies, leads me to think that a "word" is a string that either:

1. must contain only alphanumeric characters, with at least one alphabetic character and at least one numeric character; or
2. may contain any characters, but with at least one alphabetic character and at least one numeric character; or
3. may contain any characters, but with at least one contiguous alphabetic and numeric character pair in any order.

The other replies seem to lean toward alternatives 2 and 3 above. My own first guess was for alternative 1, as in the last code examples below:

>perl -wMstrict -le
"my @lines = qw(abc 345  a1 1a a1a 1a1 abc1 1abc  a1==a1 a==1);
 printf '@lines: ';  printf qq{'$_' } for @lines;
 print qq{\n};
 ;;
 printf 'and 1: ';
 printf qq{'$_' } for grep { /[[:alpha:]]/ && /\d/ } @lines;
 print '';
 ;;
 printf 'regex 1: ';
 printf qq{'$_' } for grep m{ [[:alpha:]] \d | \d [[:alpha:]] }xms, @l
+ines;
 print qq{\n};
 ;;
 ;;
 printf 'and 2:   ';
 printf qq{'$_' } for grep { !/[^[:alnum:]]/ && /[[:alpha:]]/ && /\d/ 
+} @lines;
 print '';
 ;;
 my $al_num = qr{ [[:alpha:]] \d | \d [[:alpha:]] }xms;
 printf 'regex 2: ';
 printf qq{'$_' } for grep m{ \A [[:alnum:]]* $al_num [[:alnum:]]* \z 
+}xms, @lines;
 print qq{\n};
 ;;
 ;;
 printf '@lines as was: ';
 printf qq{'$_' } for @lines;
"
@lines: 'abc' '345' 'a1' '1a' 'a1a' '1a1' 'abc1' '1abc' 'a1==a1' 'a==1
+'

and 1: 'a1' '1a' 'a1a' '1a1' 'abc1' '1abc' 'a1==a1' 'a==1'
regex 1: 'a1' '1a' 'a1a' '1a1' 'abc1' '1abc' 'a1==a1'

and 2:   'a1' '1a' 'a1a' '1a1' 'abc1' '1abc'
regex 2: 'a1' '1a' 'a1a' '1a1' 'abc1' '1abc'

@lines as was: 'abc' '345' 'a1' '1a' 'a1a' '1a1' 'abc1' '1abc' 'a1==a1
+' 'a==1'
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