Your issue is unclear to me - please read I know what I mean. Why don't you?. Specifically, providing input, expected output and a full script that replicates your problem would be helpful. Given that you are apparently not using strict, I suspect the reason you are having trouble with named variables is typographic in nature. See Use strict warnings and diagnostics or die.

foreach $keye (@key_en)
{
$linb=`cat /home/vikash/pro_1/en_1000 | grep --regexp '$keye'`;

print "$linb\n";
}
[download]

and

foreach (@key_en)
{
$linb=`cat /home/vikash/pro_1/en_1000 | grep --regexp '$_'`;

print "$linb\n";
}
[download]

are functionally equivalent. Why do you think it doesn't work?

1 more thing i had to ask is how many $_ variables can be used simultaneously and how.

There can only be one variable of any name at a given location in a script, so there is only one variable named $_.

As a side note, there is no reason to pipe cat into grep. You can save characters and possible confusion by writing that as

$linb=`grep --regexp '$keye' /home/vikash/pro_1/en_1000`;