This works for me:

my $substring = 'GATC';
my$i=0;
my$count;
my $sequence;

while ($sequence=<DATA>){
    foreach($sequence =~ /($substring)/g) { 
        print "$1\n";
         $count++;
    }
}
    print "There are $count negative numbers in the string\n";
    
__DATA__
GAGAGACCCCGATCGAGAGACCCGATCFGAGAVCTGATCCCC
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