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my $pos=0;
foreach my $sbstr ( @$array ) {
...
       substr ($nstring, $pos, length ($sbstr)) = $sbstr;
       $pos++
}

my %after;
foreach my $sbstr ( @$array ) {
...
       substr ($nstring, $pos, length ($sbstr)) = $sbstr;
       $after{$sbstr}=$pos+1;
}