use strict;
use warnings;
use Data::Dumper;

my $file = '/path/to/your/file/here.fasta';
open (my $fh, '<', $file) or die "Could not open file '$file' $!";

my (%sequence_hash, $header);

while ( my $line = <$fh> ) {
    chomp($line);
    if ( $line =~ m/^>(.*)/ ) {
        $header = $1;
    }
    else {
        $sequence_hash{$header} .= $line;
    }
}

# look at the header and sequence
print Dumper(\%sequence_hash);

my @translated_sequences;

foreach my $seq ( keys %sequence_hash ) {
      $sequence_hash{$seq} =~ s/t/u/ig;
      push @translated_sequences, $sequence_hash{$seq};
}

print Dumper(\@translated_sequences);