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while ($string =~ /abc/ig) {
    my $ins = sprintf(" def(%s)", $x++);
...
    $string =~ s/\G/$ins/;
    pos($string) = $pos + length($ins);
}

while ($string =~ /abc/ig) {
    my $ins = sprintf(" def(%s)", $x++);
...
    substr($string, $pos, 0, $ins);
    pos($string) = $pos + length($ins);
}

$string =~ s/(abc)/  sprintf("%s def(%s)", $1, $x++)  /eig;