Or, using the Bio::SeqIO, Bio::Tools::SeqStats modules from the Bio::Seq distribution, you can come up with this, (though more useful for less trivial tasks as the one here).

#!/usr/bin/perl
use strict;
use warnings;
use 5.014;
use Bio::SeqIO;
use Bio::Tools::SeqStats;

my $in = Bio::SeqIO->new (-fh => \*DATA, -format=>'fasta');

while(my $seq = $in->next_seq() ) {
    my $seq_stats = Bio::Tools::SeqStats->new(-seq => $seq);
    my $count = $seq_stats->count_monomers();
    print "Count: A $count->{A} T $count->{T} G $count->{G} C $count->
+{C}\n";
}

__DATA__
>NR_037701 1
aggagctatgaatattaatgaaagtggtcctgatgcatgcatattaaaca
tgcatcttacatatgacacatgttcaccttggggtggagacttaatattt
aaatattgcaatcaggccctatacatcaaaaggtctattcaggacatgaa
ggcactcaagtatgcaatctctgtaaacccgctagaaccagtcatggtcg
gtgggctccttaccaggagaaaattaccgaaatcactcttgtccaatcaa
agctgtagttatggctggtggagttcagttagtcagcatctggtggagct
gcaagtgttttagtattgtttatttagaggccagtgcttatttagctgct
agagaaaaggaaaacttgtggcagttagaacatagtttattcttttaagt
gtagggctgcatgacttaacccttgtttggcatggccttaggtcctgttt
gtaatttggtatcttgttgccacaaagagtgtgtttggtcagtcttatga
cctctattttgacattaatgctggttggttgtgtctaaaccataaaaggg
aggggagtataatgaggtgtgtctgacctcttgtcctgtcatggctggga
actcagtttctaaggtttttctggggtcctctttgccaagagcgtttcta
ttcagttggtggaggggacttaggattttatttttagtttgcagccaggg
tcagtacatttcagtcacccccgcccagccctcctgatcctcctgtcatt
cctcacatcctgtcattgtcagagattttacagatatagagctgaatcat
ttcctgccatctcttttaacacacaggcctcccagatctttctaacccag
gacctacttggaaaggcatgctgggtctcttccacagactttaagctctc
cctacaccagaatttaggtgagtgctttgaggacatgaagctattcctcc
caccaccagtagccttgggctggcccacgccaactgtggagctggagcgg
[download]

Output:

C:\Old_Data\perlp>perl t7.pl
Count: A 244 T 311 G 232 C 213

C:\Old_Data\perlp>
[download]