"in this case it should ideally throw some error but it just considers > a,b,c... as 0 ? why does it do that"

Adding use strict; use warnings; is advisable. See strict, warnings:

#!/usr/bin/perl

use strict;
use warnings;

my @a=qw( 1 2 3 4 a b c g f);
    
my $s=0;

foreach my $i(@a){
    $s+= $i;
    print "i: $i - s: $s\n";
}
print $s;
[download]

shows:

D:\>perl derp.pl
i: 1 - s: 1
i: 2 - s: 3
i: 3 - s: 6
i: 4 - s: 10
Argument "a" isn't numeric in addition (+) at derp.pl line 10.
i: a - s: 10
Argument "b" isn't numeric in addition (+) at derp.pl line 10.
i: b - s: 10
Argument "c" isn't numeric in addition (+) at derp.pl line 10.
i: c - s: 10
Argument "g" isn't numeric in addition (+) at derp.pl line 10.
i: g - s: 10
Argument "f" isn't numeric in addition (+) at derp.pl line 10.
i: f - s: 10
10
[download]

This is not as much a matter of variables, as a matter of the + operator in Perl. It treats both its arguments as numbers, and anything that starts with a letter will be treated as numerically 0 for that operation.

in this case it should ideally throw some error...

Further to Laurent_R's reply: If jabirahmed really wants to make these missteps into (fatal) errors rather than just having them generate nagging warnings, that can also be done. See perllexwarn. (Note that this control of fatality is lexical!)

>perl -wMstrict -le
"print 4 + 'a';
 ;;
 use warnings FATAL => 'numeric';
 print 5 + 'b';
"
Argument "a" isn't numeric in addition (+) at -e line 1.
4
Argument "b" isn't numeric in addition (+) at -e line 1.

>perl -wMstrict -le
"print 4 + 'a';
 ;;
 use warnings FATAL => 'all';
 print 5 + 'b';
"
Argument "a" isn't numeric in addition (+) at -e line 1.
4
Argument "b" isn't numeric in addition (+) at -e line 1.
[download]

(Many Perlers favor escalating  'all' warnings to fatality. I tend to agree with this practice, especially for modules, both OO and non-OO. However, I must admit that such fatality can get annoying during debugging.)

$ perl -e 'print 4 + "a"'
4

$ perl -e 'use strict; print 4 + "a"'
4

$ perl -e 'use warnings; print 4 + "a"'
Argument "a" isn't numeric in addition (+) at -e line 1.
4

$ perl -we 'print 4 + "a"'
Argument "a" isn't numeric in addition (+) at -e line 1.
4
[download]