The problem here is overlapping matches. The space between the first and the second occurence of /a/i is part of both possible matches. Since it is gobbled by the first match, the second /a/i doesn't match.

It is possible to "rewind" your regex by assigning to pos. See 'pos' in perldoc.

#! usr/bin/perl
use strict;
use warnings;

$_=' A a A ';
while (/(\s)(a)(\s)/ig) {
    print "The text matches the pattern '$&' at position " . pos() . "
+.\n";
    # uncomment this to rewind
    #pos() = pos() - 1;
}
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