my $ERR_FILE = q{/tmp/error_message.txt};

qx{COMMAND 2>$ERR_FILE};

if ( -f $ERR_FILE ) {
    open my $FH, '<', $ERR_FILE;
    my @err_msg = <$FH>;
    close $FH;
    unlink $ERR_FILE or die "Can't remove $ERR_FILE: $!";
}