use strict;
my $stdInfo="stdInfo";
print " Enter the number of student that you want to delete\n";
my $std = <STDIN>;
chomp $std;
#open file and read it into an array...
open(FILE, "$stdInfo") || die "couldn't open $stdInfo\n";
my @filedata = <FILE>;
close (FILE);

#open file for writing...
open(FILE, ">$stdInfo") || die "couldn't open $stdInfo\n";
foreach my $line (@filedata) { 
 print FILE $line unless  ($line =~  /^$std/);
}
close (FILE);
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If you are doing anything like this to a large file consider using a database or tieed file.