but somehow if loop block is visited even if data is numeric

First off you are not doing an apples to apples comparison in your conditional. Secondly you are trying to do pattern matching in a dubious fashion. Check out the following:

$ cat torres09.pl
#!/usr/bin/perl -w 
use strict;


while(my $line=<DATA>){
        chomp $line;
        if ( $line =~ m/^\d+$/){
                printf "%s\n",$line;
        }
}


exit(0);
__END__
0123
0444
A123


$ perl torres09.pl 
0123
0444
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