open FILE, "mRNA.fa" or die $!; # Opens mRNA, assoicates FILE

##</code><code>##

perl <programName.pl> <input file>

##</code><code>##

my $file = $ARGV[0];
open (my $mRNAfile, "<", $file) or die $!; 
my @mRNAdata = ( ); # initalizes the variables of array
@mRNAdata = <$mRNAfile>;

##</code><code>##

"Number found where operator expected at - line 1, near "Sequence 1" (Do you need to predeclare Sequence?).