use 5.014;
my $sampleids = ['A','_B','C'];
my @temp_SAMPLEIDS;
foreach my $var (@$sampleids) {
    push(@temp_SAMPLEIDS, $var =~ s/_//gr);
}

# OR
my @temp2_SAMPLEIDS = map {s/_//gr} @$sampleids;

use Data::Dumper;
print Dumper($sampleids, \@temp_SAMPLEIDS, \@temp2_SAMPLEIDS);

__END__

$VAR1 = [ 'A', '_B', 'C' ];
$VAR2 = [ 'A', 'B', 'C' ];
$VAR3 = [ 'A', 'B', 'C' ];