You where not clear on what you did not want to match, but maybe something like this is what you want.

my @filedata = ( '24..4 foo bar', '12..13 13..14 14..15 15..1 15..2 14
+..3 13..16 16..4 16..17 17..5 17..18 18..6 18..7 12..19 19..20 20..8 
+20..10 19..9 12..11', 'junk');

my @result;
for my $line (@filedata){
    if($line =~ m/^ (?: \s* \d+ \. \. \d+ \s* )+ $/x ){
        @result = split ' ', $line;
        last;
    }
}
print join ', ', @result;
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