Further to jwkrahn's reply, consider:

>perl -wMstrict -le
"my $str = '123 456 789';
 print qq{1st match: $1} if $str =~ m{ (\d{3}) }xmsg;
 print qq{2nd match: $1} if $str =~ m{ (\d{3}) }xmsg;
 print qq{3rd match: $1} if $str =~ m{ (\d{3}) }xmsg;
"
1st match: 123
2nd match: 456
3rd match: 789
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Try those matches without the  //g modifier.