When I try to run the code you posted, I only get these errors:

C:\>perl -w tmp.pl
Name "main::IN" used only once: possible typo at tmp.pl line 8.
Use of uninitialized value $id in seek at tmp.pl line 10.
seek() on unopened filehandle In at tmp.pl line 10.
readline() on unopened filehandle In at tmp.pl line 11.
Use of uninitialized value $name in concatenation (.) or string at tmp
+.pl line 3.
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Please try to make sure that the code you post actually exhibits the problem you are trying to diagnose.

When I change your code to the following:

my $id= shift;
my ($name) = seekName($id);
print "$name\n";

sub seekName
{
    my ($id) = $_[0];
    open (IN, "<:utf8", $0) or die "Can't open: $!";
    
    seek IN, $id, 0;
    my $line = <IN>;
    return $line;
}
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... it assigns $name a value depending on what number I give on the command line. So I would assume that this works and does not exhibit the problem of always printing 1.