Yes, that is misunderstanding the role of the g modifier. Simplistically /g means "match as many times as you can". In a list context that means the regex will return all the matches it finds. Consider:

my @matches = '1 foo 22 bar 3' =~ /\d+/g;
print "@matches";
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Prints:

1 22 3
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In scalar context however it returns true while there is a "next" match. To see what was matched we now have to capture the bit we are interested in:

while (my $match = '1 foo 22 bar 3' =~ /(\d+)/g) {
    print "$1 ";
}
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which generates the same output as above. Your code is rather like this last version except that you have "unwound" the loop.

To get the behaviour you expected without the /g you need to "anchor" the match at the start of the string using ^:

my @matches = '1 foo 22 bar 3' =~ /^\d+/g;
print "@matches";
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which prints '1'. For further regex reading see perlretut, perlre and perlreref.