In scalar context such as supplied by index, a successfully matching regex will return 1, which will be stringized to '1' for the substring for which to search:

>perl -wMstrict -le
"my $str = 'abc1def';
 $_ = 'xyz';
 ;;
 print 'index: ', index $str, /(\w+)/;
 print qq{captured: '$1'};
"
index: 3
captured: 'xyz'
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An unsuccessful regex match returns the empty string in scalar context.