Thing is, if you declare the sub outside of the ?{...} code, that code will bind the first instance of the subroutine. Printing the coderefs shows what's going on:

sub foo {
    my $window = "a b X20 c X5 d e X17 X12";
    my @o;
    my $push_o = sub { push @o, @_ };
    print "subN=$push_o\n";
    my @m = ($window =~ m/(X\d+(?{print "sub1=$push_o\n"; $push_o->(po
+s)}))/g);
    print join(" ", "Matches:", @m, "\n");
    print join(" ", "Offsets:", @o, "\n\n");
}
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Output:

subN=CODE(0x8119060)
sub1=CODE(0x8119060)
sub1=CODE(0x8119060)
sub1=CODE(0x8119060)
sub1=CODE(0x8119060)
Matches: X20 X5 X17 X12 
Offsets: 7 12 20 24 

subN=CODE(0x8131ac0)
sub1=CODE(0x8119060)
sub1=CODE(0x8119060)
sub1=CODE(0x8119060)
sub1=CODE(0x8119060)
Matches: X20 X5 X17 X12 
Offsets: 

subN=CODE(0x8119d50)
sub1=CODE(0x8119060)
sub1=CODE(0x8119060)
sub1=CODE(0x8119060)
sub1=CODE(0x8119060)
Matches: X20 X5 X17 X12 
Offsets:
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Despite three subroutines being created (--> "subN=..."), the one actually being called within ?{...} is always the same, first one (--> "sub1=..."). This sub in turn is bound to the first instance of the variable @o ...