Sleep with count down display
Naming

The names are very poor. Consider these renames:


n -> seconds
t -> unit
x -> input


Make the most out of regex

If you only allow "s", "m", or "h" as units, then instead of [a-zA-Z] you could use [smhSMH] in the regex.

Formatting

The formatting is really too compact. It's recommended to put spaces around operators.

use strict

It's recommended to use strict always.
It can help catching bugs.

Alternative implementation

Applying the above suggestions (and a bit more), consider this alternative implementation:

use warnings;
use strict;

my $seconds; 

# input could be 1, 1s , 1m or 1h
my $input = $ARGV[0]; 
if ($input =~ /^([0-9]+)([smh]?)$/i) { 
    $seconds = $1;
    my $unit = lc($2); 
    my $multiplier = 1;
    if ($unit eq "h") {
        $multiplier = 3600;
    } elsif ($unit eq "m") {
        $multiplier = 60;
    }
    $seconds *= $multiplier;
} else {
    die;
} 

$| = 1;  # force flush output on every write
while ($seconds) { 
    print $seconds; 
    sleep 1; 
    my $len = length($seconds); 
    print "\r", " " x $len, "\r"; $seconds--;
}