If you assume that the last line has the highest ID you can:

my $line;
open FH,"file.ext" or die $!;
$line=$_ while(<FH>); close FH; #now the last line is in $line
my($id)=split(/\|/,$line);
#now you can use $id+1
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If the highest ID can be anywhere in the file:

my $id;
open FH,"file.ext" or die $!;
while(<FH>){
my($nid)=split(/\|/);
$id=$nid if $nid>$id;
}
close FH;
#and again you use $id+1
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