not really sure why the last match doesn't get included or why the first one is undef

I think the capture group needs to be closed before being able to push its value.  This would work without further ado:

$data =~ /^(?:(\d+)\s*(?{ push @results, $^N }))+$/;
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(Update)  P.S.: if you use this construct in a loop like in the OP's case, you need to declare the lexical @results outside of the loop for it to work properly.  I.e., while this is ok:

my @results;
while (<DATA>) {
    @results = ();
    /^(?:(\d+)\s*(?{ push @results, $^N }))+$/;
    print "line $.: ", join('-', @results), "\n";
}

__DATA__
1 2 3 4 5
2 3 4 5 6
3 4 5 6 7
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output:

line 1: 1-2-3-4-5
line 2: 2-3-4-5-6
line 3: 3-4-5-6-7
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the following would work only once:

while (<DATA>) {
    my @results;
    /^(?:(\d+)\s*(?{ push @results, $^N }))+$/;
    print "line $.: ", join('-', @results), "\n";
}
[download]

output:

line 1: 1-2-3-4-5
line 2: 
line 3:
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(Fixed /^(:?... typo — thanks eric256! )