There is a CPAN module DataExtract::FixedWidth for handling the fixed unpacking details. I tried it on the test sample and it seems to work fine.

use strict;
use warnings;

use DataExtract::FixedWidth;
use Data::Dumper;

my $de = DataExtract::FixedWidth->new({
    header_row => scalar(<DATA>)
});
while (my $l = <DATA>) {
    my $fields = $de->parse($l);
    print Dumper($fields);
}

__DATA__
StudentId Name      Dept address                 city
1         Chellappa CSE  22 xx-colony 2nd street coimbatore
2         Vijay     IT   23 yy colony            coimbatore
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